"I've known this guy for a long time, " Pollack tells me, sitting in the shade of a palm tree in a Cannes garden. "We met at a party in the early '80s. Mostly we talked about being in a business where you're judged mostly by people you don't know. We commiserated with each other on getting beat up by the world. He was an easy guy to talk to. He didn't have any pretensions. I was comfortable with him."

That was before Pollack won Oscars for best director and best picture with Out Of Africa. Gehry had yet to become a household name. He was best known then for a 'house-within-a-house' renovation of his home in Santa Monica, which architects regarded as a seminal work. "My wife Bertha had found the house, " Gehry, now 77, recalls. "It was comfy and had a little garden and we could afford it.

I realised I had to do something to it before we moved in, I loved the idea of leaving the house intact and not messing with it, I came up with the idea of building the new house around it."

Pollack admits not knowing what to think of it. "I didn't know Frank's architecture and I didn't care for it that much, " he says. "I used to think it was always unfinished. I kept saying when are you going to finish this?"

All that changed when Pollack was in Madrid in 1997. "I thought, wait a minute, Frank has a building opening in Bilbao. So I went there the next day. And I was knocked-out in a way I never had been with architecture. I didn't understand how it could have come from this guy. It was a wild act of the imagination I didn't understand. It looked like Don Quixote got stoned."

When Pollack got back to LA, he and his actress wife Claire Griswold went for dinner with friends.

"Frank was there with his wife. I'd some snapshots of the Guggenheim and he took about five of them away from me and they were published in a book. He was talking about the fact that suddenly he was famous and all there people asking to make documentaries about him. He's a shy guy and he was uncomfortable about it. Just casually he said 'you don't ever do documentaries, do you?' I said 'no, I don't. I don't know anything about them.'" It was another three years before Pollack agreed to do Sketches of Frank Gehry. "Somebody had put up the money, " he says. "But Frank didn't have time and I didn't have time. So we said okay, we'll work in spare time. We did it at weekends, over a period of five years. When I worked on The Interpreter, I didn't see him for a year.

But every once in a while I'd take the camera and go over and talk to him for a little bit.

"Making movies for me is very hard. It takes me over two years to make a feature. I wish I worked faster. Maybe I'm lazy. It's silly to have worked as long as I have and only have 19 features to my credit.

Of course I've produced 31 movies.

It doesn't require the same personal dedication, because you have a director. I can produce more than one at a time. Right now I'm in post-production as a producer with four movies with my partner Anthony Minghella, and we're planning a present-day American version of The Lives Of Others."

Pollack never had any ambition to be a director but was accepted by the New York Neighbourhood Playhouse, where he was taught dance and acting. This lead to John Frankenheimer hiring Pollack as a coach of actors on live TV. "Then Burt Lancaster started calling up people, telling them, come and meet this kid, I think he's director material. So I was tried out on a couple of episodes of a TV series, " he says.

Pollack worked his way through the Hollywood genres, but always in terms of relationships . . . "I've managed to worm a love story into every single one of my pictures, somehow" . . . so there's some logic in now trying his hand at a documentary.

"Everybody says, 'You did this for your friend Frank Gehry, ' but I'm not that good a person, " he says. "I would say no to a lot of friends on that much work. I did it because I was curious about how Frank could do what he does. It was really my way of going to school, my way of learning something I didn't know much about."| He hadn't intended to appear in Sketches of Frank Gehry himself. "I only had my little mini-DV hand camera when I was with him. I'm used to the fact that in order to make an edit or a cut I need more than one angle. So I took my producer Ultan Guilfoyle . . . he's Irish, like you . . . and said you've got to get me a cutaway so that I can edit this. So he got a camera and started shooting while I was shooting Frank. I said, 'Don't shoot me, I'm only asking these questions to get Frank talking.' But he kept shooting the two of us. And then the editor fell in love with the footage and started cutting it in. I kept screaming ever time they would show to me, 'You can't do this, this is narcissism.' But Frank started saying, 'You know what's great is we're having a conversation, we're talking about this stuff.' So gradually I started getting used to the idea, although I'm still self-conscious about it."

Whether by accident or design, the impromptu conversational format works beautifully. The whole process of architecture is demystified. So there's Gehry in his office in a Santa Monica warehouse and his saying why does everything have to be a cube or a circle or a pyramid? Put the pyramid on top and it's the roof, put the circles and they're columns. Then he takes a piece of paper, crumples it in his hand and drops it on the floor. But there's no way you can put that in a blueprint, he says.

Solving it problem became his big breakthrough. "One of the guys who worked for him said, you know you need to work out something like that with a computer, " says Pollack. "They went to the French company Dessault Mirage, which uses a computer to design their aeroplanes, and they licensed it from them, writing a programme for architecture over the top of the hardware. And that's how he did Bilbao and the Disney Concert Hall, and how he's doing the Marques de Riscal winery in Spain. So now he can really twist the pieces of paper and say, 'I want to make that.'" Gehry claims to be almost computer illiterate. He doesn't know how to turn on the DVD. He can barely use the technology in his car. He comes up with sketches of his ideas, but leaves the physics and the engineering to a team of 150 people who work for him. His curiosity is his genius. "I drove him crazy at the opening in Bilbao, " says Pollock. "I dragged him around asking questions. For instance why did he cover the building with iconic glittering titanium? Well, he said, 'I took a lot of different metals and I put them outside and let them stay for three months to see what happened to them. I would come and look at them in the morning and in the afternoon, and in the evening and then when it rained, to see what was happening.'" Eventually Gehry decided titanium would work best. "It was so horribly expensive that everybody laughed, " says Pollock. "But he got this titanium company interested and by making it very thin was able to afford it. It's so thin that when the wind blows it billows like cloth on the outside."

Pollack, who is 73, is unlikely to make another documentary. But the experience of filming Sketches Of Frank Gehrymay feed into his fiction movies. "In a fiction movie I work always with a script, " he says. "But with this I found when I wrote questions down it was boring. It was like canned food. But if I started talking to Frank it was different. The conversation would go where it was going. It's why the shots are so bad sometimes. I couldn't pay attention to the camera. Maybe I'll find a way of taking that back into fiction movies."

'Sketches Of Frank Gehry' opens at the IFI on 29 June