Errol Morris spent more than 30 hours interviewing Donald Rumsfeld. He sifted through thousands of memos from the former secretary of defense and architect of the Iraq war.

Yet at the conclusion of so much investigation into the mind of Rumsfeld, Morris says he's left with nothing but the grin of a Cheshire Cat. Morris calls his latest, thickly ironic documentary, "The Known Unknown," a "fable" about a man stuck "in some Looney Tunes world of his own devising."

"The Known Unknown," which opens in theaters Friday, is in stark contrast to Morris' Oscar-winning documentary "The Fog of War," about another wartime secretary of defense, Robert McNamara. Whereas McNamara was remorseful about the Vietnam War, Rumsfeld's views on Iraq and Abu Ghraib are largely unchanged.

In a recent interview over coffee, Morris was still rattled and mystified by Rumsfeld.

AP: Some have said your film fails to reveal more about Rumsfeld.

Morris: It doesn't matter what the question is. It's never, under any circumstances, going to be answered. I came to a conclusion about all of this - it's my hunch - that he's not hiding anything. He's hiding the fact that he's hiding nothing. To me it's a horror movie.

AP: His evasive replies can often be witty and almost comical.

Morris: They're comical except that they involve war, the terrible loss of life, the destabilization of entire areas of the world. Both Iraq and Vietnam for me - I came of age during the Vietnam War - and not to minimize the death and destruction, but part of it for me was the destruction of an idea about America.

AP: Though you've done considerable research, this is - like many of your movies - a first-person film. You seem to believe strongly in the power of the interview to unlock mysteries.

Morris: It can be, in part - and this has always fascinated me - an interior monologue. It's not quite interior because it's being spoken. A friend of mine years ago said to me, "You can't trust anyone who doesn't talk a lot because how else would you know what they're thinking?" Well, maybe that's true, maybe it isn't. But how people describe themselves is a powerful way in. It's a powerful way in to figuring out how they see the world, who they are.

AP: In the 1970s, you interviewed Ed Gein, the inspiration for "Psycho," at a psychiatric hospital in Wisconsin, and you made your first film about a pet cemetery. What drew you to such subjects?

Morris: My father, who was a doctor, died when I was very, very young. I was not even 3 years old when he died. I don't remember him. Everybody would tell me stories about my father. There were pictures of my father everywhere around the house. Many of the stories were contradictory. There's a mystery, a really powerful mystery about someone you can't remember but who was such a central part of your life. I think there were a lot of ingredients there. Concern with the past. My project on Ed Gein was and still will be (titled) "Digging Up the Past."

AP: You seem to have an obsessive nature.

Morris: I have these twin obsessions, I'll fess up: pursuit of truth and the avoidance of truth. A lot of people have said, "Why bother examining this? It's over and done with." I don't agree with that sentiment at all. I think it's really important to examine it. There's probably more to be learned from untruth than truth, from false belief than from true belief. Maybe there's more to be learned from nonsense than from non-nonsense.

AP: How would you compare "The Fog of War" and "The Unknown Known"?

Morris: (Rumsfeld) gave me a lot of his time. He probably would have given me even more. He's charming. I like him. And yet in the end, there's this "but..." that hangs at the end. In the end, I'm left frustrated, dissatisfied and I would have to confess angry. He should be able to do better, and the fact that he was not, given every opportunity to do it, is really frightening. If people want to think of this as my failure to properly interview Donald Rumsfeld, I would say it's Donald Rumsfeld's failure to be properly interviewed by me.

AP: Your final question to Rumsfeld is why he granted the interviews. He laughs and says he doesn't know. Why do you think he did?

Morris: In the end, I think it's vanity. Total self-confidence. You could call it a kind of fearlessness. I spent 20 hours with Robert McNamara, who was far less cooperative than Rumsfeld, far more difficult. But I was dealing with such a different kind of man, really couldn't be any more unlike Rumsfeld. A man reflective and regretful. A man trying in some way to reckon with his past. And then on the other hand, with Rumsfeld, a man who reckons with nothing.