When you get out of a car, feeling sick, tired and sweaty after hours of travelling down bumpy, dusty roads in India, there’s one thing guaranteed to make you feel much worse, and that’s having an A-list Hollywood star get out of the car with you – dressed casually, to be sure, in jeans and locally bought cotton shirt and scarf, but otherwise looking as radiant as she appears on-screen, and seemingly unaffected by the heat or the journey.

Two years ago, I went with Scarlett Johansson on a gruelling two-week tour of India and Sri Lanka, visiting Oxfam projects in her then-new role as an ambassador for the charity. Few of the people me met knew she was a movie star in the West, but all of them commented on her beauty – and her empathy. She was a great travelling companion, with an endless supply of good music on her iPod, lots of stories and jokes, and a long, dirty laugh when other people told good ones. She was always willing to stop off at a roadside stall for sweet chai tea and deep-fried Indian snacks, or to invite the Oxfam crew up to her room at the end of a long day for a glass or red wine. But most of all, she was genuinely interested in the people we were there to meet, asking intelligent questions, listening to their stories, comforting people who were in distress and treating their homes in city slums and desperately poor rural areas with the utmost respect.

She won’t thank me for telling you this, but afterwards she not only made large donations of her own, but also worked behind the scenes, writing to other stars, organising fundraising dinners and using her fame to help the people she had met. When you see Johansson wearing expensive jewellery on the red carpet, for instance, it is usually in exchange for a donation by the jewellery company to the charities she supports.

In many ways, her friendship with Bono is an unlikely one. Johansson is just 25; Bono is 49. She has only recently married Canadian actor Ryan Reynolds; Bono met his wife Ali Hewson at school, they have four children. What they have in common, though, is a passion for music. Johansson may be one of the best actors of her generation – she will be seen next in Iron Man 2, out this spring – but she has also released two albums. In 2008 there was a well-received collection of Tom Waits covers, Anywhere I Lay My Head, and last autumn she released Break Up, a Sixties-flavoured album of duets with US singer/songwriter Pete Yorn. But what she really shares with Bono is the desire to use her celebrity to highlight the struggles of others.

Johansson was an early supporter of (RED), the organisation Bono funded with US activist Bobby Shriver to raise money for the Global Fund to Fight Aids, Tuberculosis and Malaria. Pioneers of conscious consumerism, Bono and Shriver persuade big brands to make (RED) products, from trainers and T-shirts to iPods and credit cards. Every time you buy one of these products, the company concerned donates a big cut of its profits to fight illnesses in the developing world. The results speak for themselves. So far, (RED) money has given more than 110,000 people with HIV or Aids access to life-saving antiretroviral drugs to reduce the chance of passing the virus to their babies. It has offered testing and counselling to 3.4 million people, and supported care projects for children orphaned by Aids.

When I catch up with them, both Bono and Johansson are in New York. This is where Johansson grew up, and when she’s not working, it is still her favourite place to be. It’s a few days before a concert at Carnegie Hall in Manhattan to raise funds for (RED), in which a variety of big names – including Bono, Johansson, Courtney Love and Rufus Wainwright – are due to perform songs with and by Irish singer/songwriter Gavin Friday. At first, their conversation is all about which Gavin Friday song she will perform at the benefit, though Johansson – a genuine U2 fan – is also keen to talk about the band’s appearance on Saturday Night Live the previous weekend. ‘We don’t normally like doing TV,’ says Bono, laughing, ‘because it gives people the ability to turn us down. Which we don’t do at concerts.’

It’s easy to see why Bono has managed to meet everyone from presidents to heads of huge corporations and persuade them to support his causes. He has done his research on me, for instance, and although we’ve never met before, he charmingly points out that I once put him on the cover of the magazine I edited, The Face. I remind him that he wore a dress for the pictures inside, and he laughs out loud. ‘A dress?’ says Johansson, amused.

‘I can’t quite explain how the dress got on for the photo session,’ says Bono. ‘We thought it was very funny at the time. I think there was a beer in my hand. I looked like a builder who’d pinched somebody’s washing off their line.’

‘I’ve worn a tuxedo several times,’ says Johansson, ‘and I have to say, it always went down pretty successfully. A girl looks great in a tuxedo, but a man in a dress? It all depends how big your boobs are, I guess!’

Bono: So, Scarlett, when (RED) was just a booklet with four logos, you became the first star to endorse it. And three years later, we’ve raised nearly $137 million, and 100 per cent of it goes to buy HIV and Aids drugs and support. It’s just mind-blowing. But you took a risk, supporting us then. Your empathy, is that a Danish thing? Because Denmark is one of the most generous countries in the world, in terms of the aid they give.

Scarlett Johansson: It’s not to do with my heritage, necessarily. My dad is Danish, but I’m American. I was born and raised in New York, and one of the wonderful things about this city is you’re constantly in contact with people of every social class and circumstance. When you ride the subway, you could be sitting next to someone who’s on his way to work in Wall Street, and right next to him is someone who’s been sleeping rough for years. I’ve always been curious to explore how people live, from every different walk of life. Part of that is being an actor, but also my parents always encouraged us to accept all different kinds of people. I don’t remember when you and I first started to talk about the (RED) idea of conscious consumerism. Were we at a dinner party?

B: I think it was your brother’s birthday. In fact, he’s your twin so it would have been your birthday, too.

SJ: I remember you’d just signed the first deals with big companies: with American Express, and maybe Apple or Converse. What sparked your interest in Africa, particularly?

B: I went to Ethiopia years ago. I don’t even know if you’d have been born! It was after Live Aid. I went with Ali for about a month. We worked in an orphanage, and we didn’t tell anyone we went there. I was known as ‘the girl with the beard’ because I had an earring.

SJ: [Laughs] So they knew early on, before you put on the dress!

B: I wrote plays to keep up the morale of the kids, and to educate the adults. One was about giving birth: some of the local traditions were very unhygienic, and there were a lot of fatalities. The kids would put on the play while the adults were standing in line at the food shelter. I had a great time, and I left Africa changed. But my heart was broken. A man gave me his child, and begged me to take him home to Ireland. ‘If he stays with me, he’ll die,’ he said. ‘But if my son goes with you, he will live.’

SJ: I have goose bumps from that.

B: I do, too. We didn’t bring the boy home with us, and I can’t even remember his name. But we’ve talked about him a lot, Ali and I, and he’s kind of been a part of this. On your trip to Rwanda with (RED) last year, do you remember a girl called Denise?

SJ: Yes, of course. I visited her family.

B: I remember her two years ago, when she was so thin she was on death’s door, and we got her the drugs. How is she now?

SJ: It was explained to me, this idea of the ‘Lazarus effect’ [with HIV/Aids], before and after taking antiretroviral drugs. But it was something else to see it. I’d seen photographs of Denise, and she was so sad and hopeless, with sunken eyes. But when I met her, she was fine. She had light in her eyes, she was a healthy weight, and she was able to look after her brothers and sister. She was happy to receive us, and kind of shy because the whole village came to see what was going on. The difference was amazing. She was so ill, and now she’s able to go to school and live a normal life.

B: It is incredible. Two pills a day, and you can move from death’s door to being back at work in 40 days. Lots of people don’t know this, and once you do know, it’s so exciting! You just feel: ‘Of course we can get them two pills! We can keep somebody alive!’ When we started out, the medication was really expensive, about £60 a day. But because of the Global Fund, and work done by president Clinton in lowering prices of generic, or unbranded, drugs, the price has come down to just 25p a day.

SJ: There’s still such a taboo around HIV and Aids in parts of Africa, so even when people do get well, they’re living with this secret. At the hospitals, I was really touched by the support groups for people living with HIV and Aids. They can talk about how to deal with their schoolmates, or how to have a relationship. Kids were asking their counsellor: ‘Will I ever be able to get married?’ or ‘Will I be able to have children?’ Education on these issues is as important as receiving the treatment. It keeps the spirit alive.

B: You start with one issue, like HIV/Aids. But then you start to see the social conditions that the disease thrives in, and education becomes an important part of it. Especially girls’ education. Girls who are in school are half as likely to get infected. Poverty makes women vulnerable. It incensed me to discover that the most vulnerable group in Africa is married women. Their husbands are migrant workers, and they bring the disease back home. Does the unfairness to women annoy you?

SJ: I’ve always found that women have power in numbers. We draw strength from the support of other women. In some cultures, girls can be seen as a burden to a family because of the need to pay out a dowry when they marry, and they are vulnerable to all kinds of abuse. When I visited shelters for female victims of violence in India, I found that a lot of the women had been raped, even at home. That’s still very taboo. Even in the United States, it’s hard to talk about being sexually abused by your husband or your boyfriend. So being able to find solace in support groups makes a huge difference. What I found amazing in Rwanda was that since the genocide, the Rwandan parliament has become the first in the world where women are the majority: 56 percent of the MPs are female. When I visited the Ministry of Health there, I met so many educated, inspiring women.

B: [US music mogul] Andre Hareel said to Bobby Shriver, who founded (RED) with me: ‘You and Bono need to be like the lions of Africa, just go sit in the shade because we’re all getting sick of the sight of you. What you really need is some lioness energy!’ He was being playful, but it’s true – I think women talk about these issues with more potency. I love your tenderness towards the people you met, and you don’t even have kids yet. Because that certainly switched on Ali, when she became a mother: having our kids meant she really identified with other kids. And I know that Bazaar readers will really take this interview on board, far more that I think the readers of a men’s magazine would. In your travels with Oxfam, didn’t you focus on women’s rights?

SJ: When I went to India and Sri Lanka, my trip focused primarily on education, and on women’s support. In Sri Lanka, the tsunami was disastrous to fishing communities. Many of the men were out in their boats and never came back, leaving the women unable to provide for themselves or their children. Oxfam offers small loans to women wanting to start their own businesses, and we met some who had thriving mushroom farms. When it came to domestic violence, poor women in India often couldn’t even turn to their own mothers because it was seen as their duty to deal with whatever their husbands did. I’m fortunate to have had a wonderful role model in my mother, who’s a very strong woman and has always taught my sister and me to be independent and have confidence in ourselves. Not everyone has that.

B: Were you worried about what you might see in Rwanda?

SJ: When I was six or seven, a very dear friend of my mother’s was a gay man who became very ill. She never sheltered us from him. We always knew what was going on, and he eventually passed away. So I never thought of Aids as being something that was catching, or taboo in any way. In Rwanda, I’d been warned that we might see disturbing things, and that some people might be close to death. So I had a bit of apprehension walking into the first hospital. But I soon realised that you’re standing before people. They might be very ill, but they’re just people like everyone else. Suddenly you’re not aware of how you’re feeling, how you’re going to look. If you let your guard down and stop being aware of yourself, you’re able to really connect with people and listen while they tell their stories. And the people from (RED) never made me feel self-conscious about who I was or what I did – we were all in it together. We were there to educate ourselves, to draw attention to (RED), and to just be very present.

B: It’s hard when you find yourself in such a harsh juxtaposition with somebody who’s fighting for their life. It used to make me feel more awkward than it does now, being this rich rock star next to a starving African.

SJ: Is that because you’ve had more experience, or just that you’re older now?

B: I don’t give a shit how things look any more. I just want to get the results, get the cheque signed. If that takes me looking like a totally unhip white messiah, I don’t care. You do whatever it takes to get people what they need to survive. For me, it was coming home that was the hardest. Coming back to my privileged life. I used to find that really difficult.

SJ: On my last night in Rwanda, (RED) put together a dinner with the Ministry of Health. We’d all become friends with some of the doctors we’d met there, and we were all sitting at this big table and laughing. It was so lovely. So when I came home, I just felt incredibly connected. Much more than before. You do gain a certain perspective when you see how people get by with so little and how they’re just happy to have simple things like a well and clean water nearby. I felt lucky to just live somewhere that has running water. Suddenly a toilet seems such a luxury. It’s miraculous! You really are thankful for the small things.

B: If you’re a songwriter or an actor, there’s a certain amount of navel-gazing. It’s just part of who we are. I found it gave me a different perspective, it took me out of myself. Your album of Tom Waits songs is amazing, but have you written any songs? Did your travels inspire you?

SJ: I don’t really write. I take pictures on these trips, and I talk about it afterwards. I’m so lucky. How many people get the chance to travel like that? It’s incredible! I met so many fascinating people, and I was able to bring their stories back with me, and spread the word in interviews.

B: We need that excitement around the issues. When I was in Washington DC, speaking to the committee deciding how much of America’s money would be given to Aids, the politicians say to me: ‘But Bono, there’s no heat on this. I’m not hearing this in my constituency.’ Now everyone’s interested. Your generation may actually get more done than ours.