Billy makes a memorable junkie

Jesus' Son is based on a novel by Dennis Johnson. I'm ashamed to admit I didn't know his work before I made the film. I've read Jesus' Son three times now.

That book alone is enough to make him my idol. The book has really grown on me, unlike any other piece of fiction I have read. There are passages that I keep going back to. One of the only other things I've done that with is Rilke's Letters To A Young Poet.

I play a heroin addict, an incredibly compassionate person, who simply has no way of dealing with his overwhelming empathy for people. He shuts himself off and uses drugs, because it's too hard to feel all that he feels.

I spent some time with a narcotics officer from the Philadelphia police department, who was also a former junkie. But my preparation was really the exercise of imagination - I wanted the drug scenes to have authenticity, but I didn't want to mimic the behaviour of somebody specific in the way I did for, say, Without Limits.

Alison [Maclean, the director] had this idea that we should put on a lot of weight. Characterisation of drug addicts tends to be an opportunity for people to lose weight, creating a very specific image in people's minds of what a drug addict is.

When you go to a methadone clinic, and speak to heroin users, they come from a wide range of backgrounds. It was important for us to build what might be a different depiction of a drug-user. It's important to avoid stereotyping them - in doing that, we dispose of a certain amount of their humanity.