For Shelter, his first film as a writer-director, Paul Bettany cast his wife Jennifer Connelly in the demanding leading role of a homeless junkie. And he let her get on with it. "Mostly I think character preparation is the actors' job," he says. "And I don't like it when a director tells me how to go about it, just as I don't tell a director where to put the camera or what lenses to shoot with. If I'm struggling as an actor I'll ask a question, but mostly I like to prepare quietly and do what I need to do to be ready."

Jennifer Connelly in ShelterConnelly's role in Shelter was particularly demanding

To prepare for their roles, Connelly and costar Anthony Mackie spent time on the streets meeting transients and talking to experts. "Jennifer became a card carrying member of New York's needle-exchange programme," Bettany says. "She would often leave me at home with the kids and off she would go to meetings or to hang out at night in parks. She also spent months gradually losing weight until she got to a pretty frightening place."

Meanwhile, Bettany got to grips with his role as a filmmaker. "Pre-production was the worst part of this experience for me," he says. "It was totally alien, a part of film production that I had never seen. I hated it. It seemed to just consist of people telling me what I couldn't do because we didn't have enough money!"

More: Bettany previously starred in 'Avengers: Age Of Ultron'

Bettany was determined not to make "an art film that just takes you to a really dark place and leaves you there and just says, 'Isn't it bad? Aren't you lucky you're not homeless?' I wanted to write a story that is uplifting - hard-won but uplifting."

For her part, Connelly found the role demanding, but also beautiful and complex, especially with her husband behind the camera. "Working with your partner, that intimacy was a different experience for me and was really very fulfilling," she says. "I thought it would be stranger than it was. In a way, it felt very familiar. We met at work. We've been on the same set; we've been on each other's sets. And it was really great to have that much trust, to have that degree of intimacy with your director, especially when the subject matter is so intense. It's an amazing feeling of freedom and safety. Because it was low-budget with only a few days to shoot, we really couldn't leave it all behind at the end of the day. We had to prep each night for the next morning. Although it was probably somewhat miserable for the kids, it actually helped that the film was being made on such a short schedule."

And Bettany was fully aware of what he was asking of her. "She goes to some deep, dark places that are also really revealing and involve nudity," he says. "But I think she trusted me as her partner, and she knew she could kick the s**t out of me for the next 20 years if I got it wrong!"