Acclaimed documentary filmmaker Errol Morris has dealt with subjects including an investigation into the wrongful arrest and conviction of an alleged cop killer, a chilling portrait of an American Holocaust denier, and a study of California's pet cemeteries. Now, in The Fog Of War, he offers a humane examination of the life and times of Robert McNamara, the controversial Secretary of Defence to presidents Kennedy and Johnson.

What interested you about Robert McNamara enough to want to make a film about him?

I wanted to make the film ever since reading McNamara's book In Retrospect, which came out in 1995. There were many reviews that came out at that time, and it was peculiar because I felt they were mostly reviewing a different book to the one that I had read. For example, the book was often described as a mea culpa or a confession, but I felt it was something more interesting. I felt he was involved in this battle with himself and history, and struggling to understand how he had become involved in this quagmire [i.e. the Vietnam War]. As a central figure in all of that, it became a very important question to examine.

Did meeting him change any long-held opinions you had?

I demonstrated against the Vietnam War years ago, and my feelings about that haven't changed at all. But my feelings have changed about Robert McNamara. There is a received view about McNamara, the man McNamara is supposed to be, that doesn't really quite fit with the McNamara that I know.

Are you referring to the ruthlessness he once displayed?

McNamara emerges in countless books and articles as a guy who was an efficiency expert, a number cruncher, who believed in reducing social problems to formula. And then, so the story goes, he gradually had misgivings about the war and by 1967 had become a dove instead of a hawk. This is a story that has been disseminated in the press, call it the 'King James version of Robert McNamara', but I think there is a very different, much more complicated story that emerges from the Johnson tapes, among other things.

Making the documentary, was there a sense of getting a life on tape, of getting a man admitting, "This is what I've learned"?

Yes, it's a very powerful part of the movie because it doesn't have multiple characters, it has one character. Sometimes it has different versions of one character, the 40-year-old McNamara talking to the 80-year-old McNamara, and vice versa, but it's still McNamara. It's not an external story, it's an internal story, because it's him going back over his life and trying to understand himself. As a result, I think it becomes part dream, part history, part self-justification, part lesson.

The Fog Of War is released in UK cinemas on Friday 2nd April 2004.