`After I'd finished the film, I didn't know if I wanted to show it, or what reaction I expected," says director Donna Deitch about her documentary An Angel On My Shoulder, screened last week at the IFC's Lesbian and Gay film festival. "I had never done anything so personal before, so I was worried about the self-revelation involved, which may sound strange because the film is supposed to be about Gwen but a lot of it is also about me."

Death on film is a commonplace, but still a mystery, usually reduced to tear-jerking banality or cartoon jokiness. When San Francisco-born Deitch set out to record the experience of her best friend, actress Gwen Welles, after Welles was diagnosed with cancer of the bowel in 1992, the two crafted a memorable and affecting portrait of what it means to die.

"She is so comfortable in front of the camera. She loves the camera and the camera loves her," says Deitch, who still mostly refers to her friend in the present tense. "She knew that there would be a sort of a future for her, through the film, a kind of immortality from doing this, which separated her in a way from other people who go through this." Using home movie footage of her own mother, who had also died of cancer, Deitch draws parallels with her previous experience of mortality. "It was very productive for me, because Gwen wanted to express herself. When my mother died, she buried and contained everything, which makes it a complete mystery. I suppose it was a generational thing, but I don't know anyone in my generation who could have done what Gwen did, or would have wanted to make a film about it. They might have been fine to talk about it, but not in front of a camera. With Gwen, it was as if she was playing herself and being an actor at the same time."

An Angel On My Shoulder is a loving yet unsentimental portrait of friendship in the face of death. Told an operation is necessary to stave off the inevitable, Welles defiantly responds: "The only surgery I believe in is plastic surgery." But the film doesn't flinch from depicting the harsh realities of terminal illness. "Gwen isn't afraid of dying in the film - she's afraid of pain," says Deitch.

"I thought that was the most frightening thing in the film, the pain rather than the dying. We're all going to die. Pain is an odd thing. If you've ever been through extreme physical pain and then it's over, it's very hard to have a memory of it. In the same way, it's very hard to describe it."

Over the course of two years, Deitch followed the optimism and depression, the hope and resignation that accompany the modern way of dying, as Welles looks for answers from different treatments. "She would get the idea for a certain kind of alternative therapy," says Deitch. "But she never stayed with it. With her macrobiotic cook, half the time he wasn't cooking, and they were just lying around, talking and laughing."

Having directed many episodes of the television series ER, Deitch is more aware than most of the fascination we all have with matters of life and death, especially when presented in glossy, digestible form. "I think one of the reasons for the popularity of that show is that it offers a vicarious experience of trauma, which most people are really fascinated by but don't want to have the direct experience of. They don't want to be on the gurney, they don't want their friends or loved ones on the gurney, but they want to know what it's like."

Deitch first met Gwen Welles in 1985, when she cast her in her debut feature film, the lesbian romance, Desert Hearts. It was one of the most critically acclaimed directorial debuts of the decade, but since then Deitch has been working mostly in television. For the past few years, she's been directing some of America's most popular and successful TV dramas, from ER to Murder One to NYPD Blue. "After Desert Hearts I got a whole onslaught of scripts that just weren't very good, and Oprah Winfrey offered me a four-hour television drama, which was a pretty good experience. As a result of that, I got the opportunity to work on some of the best material television has to offer."

But surely, I suggest, she'd also like to direct another feature film? "It has been frustrating in a way, but I don't go into work every day and say I wish I was working on a feature. I have had the opportunity of working with some of the greatest actors around. It's not as if the great actors are only working in the cinema - a lot are in television, and the writing is often hugely superior to the writing in cinema.

"The other thing which is very important is that I have learned an incredible amount about filmmaking. What often happens with independent film-makers is that their first film comes out of some grand and intense passion, a desire to drive home a message or tell a very personal story, and that's often not backed up by much personal experience. They may previously have been an editor or an actor or a lawyer or a student, but in most cases they haven't been a director. I had done a number of documentaries but I hadn't worked on a feature before Desert Hearts. "Then when people go on to make their second film, which in Hollywood often becomes a kind of assignment, there's no real passion there, and there's not enough craft to back it up." After Desert Hearts, she says, she spent a lot of time trying to get projects off the ground. "Running around to meetings and having these endless conversations that can take months or even years, and you're not actually working as a director, and that's a horrible thing."

It's not easy for anyone to make a career as a film director, but it's a particularly male-dominated world. You don't even need the fingers of one hand to count the number of female directors working for Hollywood studios at the moment, she agrees. "Of course gender is an issue. Otherwise how do you explain why there are so many more male directors than female? Do we have a gene missing?

"I actually think directing is very female. It's a job in which you have to hold everything together in a sort of family way. It's highly organisational, but it's very intimate when it comes to dealing with actors. I've tried to structure my schedule so that I spend six months a year as a director-for-hire on television, and six months trying to get my own projects off the ground. So the completion of An Angel On My Shoulder has been a real boost for me."

She has just sold worldwide television rights to the film.

"Working in television has given me a huge amount of experience, so I feel that when I go out to make my next movie, I'll have the passion, but also the craft to back that up, and I know that I'll have a lot more to bring to the table than I had 10 years ago."