I had intended to ask them to start by summing up their characters, but this proved impractical: the purpose of the long and enormously detailed preparation process is to create complex, contradictory, impossible-to-define personalities. To put you in the picture though, the sardonic Johnny (Thewlis) arrives in London broke and homeless and fetches up at the flat of the eccentric, flamboyant Sophie (Cartlidge). His subsequent tragi- comedy odyssey takes him through a city peopled by down-and-outs, loners and fringe dwellers and stalked by chaos and poverty. Now read on . .
DT: 'The first week of rehearsals is one-to-one. It's just you and Mike. Coffees are brought in, you smoke, and you give him a list of people you know of your sex and roughly your age. Obviously some people you know well and you talk about them at length. On the other hand you might come up with someone you met once in a pub two years ago. The whole process is a bit like seeing a shrink. Mike keeps a notebook but you're not allowed to write anything down at all.
'Then he'll start eliminating people from his list in your presence. Eventually he'll get it down to about five and he'll ask you to compare things like their sexuality or their class background. Their sensitivity to art - what each character might think confronted by, say, Picasso's Guernica. Finally he choses the character. It's a historic moment because you know that this is the bloke you'll be stuck with for the next six months. On Naked, when he said 'This guy is the guy', I said 'Oh, Christ,' because he's one of the most complex people I know.
'In the next few months you build on him. You go back before he's born, to how his parents met; you discuss his life year by year in detail. In improvisation, if someone says 'What's your mother's name?' you know it as if it were your own mother's name. Mike discourages you from allowing the work to enter your personal life. But you're bound to carry it with you when you go home.'
KC: 'After you've discussed and developed a character, you start to inhabit her physically, to imagine how she might move and dress and her way of speaking. It's not that you put on a voice like you'd try on a hat. It happens almost without you noticing it, the way a new shoe wrinkles the way your foot moves. You don't find someone you know and just play them. It's as if you were an Impressionist painter - you start with something you know, but it ends up unrecognisable.'
DT: 'Mike gives you a talk at the beginning in which he warns you about the paranoia that you'll suffer. Which you do. You work for three weeks, and then you might be on standby for three weeks. I had a lot of time off when I worked with him before on Life is Sweet and got terribly paranoid. He begins rehearsals in streams. He starts with two actors, then brings in two more, and then two more. If you come in at the beginning you have a good idea that you'll be substantially involved.
KC: 'You're not allowed to know anything about the story apart from what your character would know - it's not easy to place yourself, just as it's not easy in life. And it doesn't follow that, if you've done a lot of improvisation, you'll be in a lot of the film. Much of it is to establish the characters' history before the events you see.
'The more we went on, the easier it became to flip in and out, but I found I didn't want to switch off - I wanted to explore all the time what Sophie might feel about things. You don't usually get the chance to allow a character's subconscious to inform a piece of work. When you're on standby you feel a bit like a doctor on call. But those periods out can be quite creative as well - you can never rehearse or prepare enough. It's a bottomless pit.'
On the streets
DT: 'Going out in the streets as your character is the most fun because you meet ordinary people. At first you don't think anyone will take you seriously. But they do. Johnny gave me licence to do anything I liked in public, which was great.
'Often Mike will be there watching you and maybe taking a few notes. No- one ever seems to recognise him. Once a scene I was improvising with another actor broke into a physical fight. People were scared; they stopped; there were cars pulled up. Mike was watching from across the road and eventually he thought, 'I'd better stop this.' People saw this little bearded bloke come up to us, say two words and the whole situation was dissolved. They must have all wondered what he said.
'It was fortunate because the police had turned up by then and we had to go back with them to the rehearsal room and prove our credentials. In character I would have probably hit one of the policemen and I'm not sure whether it would be sufficient defence in court to say, 'It was Johnny - it wasn't really me'.
'Sometimes we meet other actors. One day Mike told me to get in costume, walk about a mile across Marylebone to a particular building, sit down in the doorway and see what happened. At the same time, he would be working with Peter (Wight), who plays a security guard, and put him in place. By the time I arrived at the building, Peter might have been waiting there for several hours. One of the scenes in the film came out of that.'
KC: 'On one occasion while I was out as Sophie a car pulled up and the man offered me drugs. That might happen to me 'as me', but it probably wouldn't. Older people would be shocked at my clothes; there would be a 'tut-tutting' following me down the street. Once I went into a fish and chip shop in Marylebone at about 8 o'clock at night and the whole shop fell silent. Those were some of my favourite times, when you were alone with a character. Sometimes Mike would be there observing, but I never felt it. He's got a skill from years of practice - it's like David Attenborough and his gorillas.'
DT: 'Some people think we improvise in front of the camera. Some think we improvise and then Mike goes off and writes the script and we learn the lines. What actually happens is that Mike does eventually go away and come back, but just with a series of scenes. For instance, 'Scene One: Johnny meets Sophie'. That might be all it says. We remember what happens between our characters from the early improvisations and we'll act it out again - it could go on for hours. An assistant takes it all down in shorthand. Then it has to be broken down into chunks for filming. We'll decide on the funniest or most pertinent lines and we fix it. Mike makes a great effort to shoot chronologically, which is rare. He views the rushes, along with his producer and the people at Channel 4, and they discuss how the story is taking shape. But obviously I'm not party to that.'
How it was for them
DT: 'I was very hyperactive and tense. My health suffered; I was smoking a lot and drinking a lot of coffee. I was reading an enormous amount, so I had this great access to words. Mike didn't ask me to lose weight for the role, in fact he got quite worried when I said I wasn't eating.
'But personally I found it very liberating and exhilarating; you never at any time felt you were being treated as a puppet, which you do with some directors. It's very cathartic - you get a lot of your own problems out on the surface. I'd be loathe to say it was disturbing because people will go round saying 'Mike Leigh fucks with your head.' Most of the time I was very happy because there was so much going on in my mind. I didn't ever not want to be Johnny. I'm not expecting Mike to ask me to be in his next film - it would be too intense. At the same time, if he did . .'
KT: 'I was awestruck when I saw the film and desperately proud to be anywhere near it. It's unique - you couldn't compare it to anything else. I'd give anything to work in that process again.'
'Naked' plays tomorrow at 7.30 in the Edinburgh Film Festival. It opens in London in October.