"This is what I really enjoy, working with film-makers," he says later, sprawled and smoking during a brief break. "You form these short-term families, it's like going to war. The movie is a battle you have to fight and win, and it very much has a mind of its own. Once you start, it's like a speeding train, you're just holding on for dear life, seeing where it takes you, and you don't get a lot of sleep, but you form these incredible bonds with other film-makers, you get ideas, and the ideas are the adrenaline, they're exciting. You become morose when you finish. You're an adrenaline junkie."
Zimmer's route to his battlefield was twisting. He left Germany "as soon as I smelt authority". He rejected the classical music his background forced on him, embracing rock'n'roll, finishing up in England, "where at least I didn't try to burn down the school". He dreamt of America, where the music and movies he loved were made. But he thought if he ever went there, he'd be swallowed up. He thought of the last shot in Elektra Glide in Blue, a cop stranded on a highway, the camera pulling back into infinity, and saw himself, "a tiny ant". So he became a pop star instead. With Trevor Horn, he formed Buggles, and wrote "Video Killed the Radio Star."
But rock musicians bored him. He was meeting more interesting people writing jingles for ads, for directors like Ridley and Tony Scott. He was starting to work, too, with Stanley Myers, the composer of The Deer Hunter. Myers' talent was orchestral, Zimmer included guitars and synths, when he heard them in his head. The collaboration made him.
His first full assignment was Nic Roeg's Eureka. He asked Roeg what he wanted. "He looked at me like I was a complete idiot, and he said, `The sound of the earth being raped, dear boy!' That was when I knew I was in the right business."
Zimmer's career since has been almost exclusively for good directors, including both Scotts. But it is James L Brooks, the director of Terms of Endearment and As Good As It Gets, who he's closest to. They formed their bond on a film which almost no-one has seen. I'll Do Anything was briefly notorious, a musical which had its songs removed, a victim of disastrous test screenings. Sometimes, Zimmer still thinks of it. It almost killed him.
"One of those years that it took, I would come home every night holding my chest saying, `I don't think I'll see Christmas,' " he remembers. "It became so overwhelming. It became incredibly personal. Because you do not want to disappoint your director, and things are not working, and the audience hates what you're doing, and it ... gets pretty severe. You do take it personally, you do take it personally when 600 people in a room go `We hate this!' By the end of it I was completely obsessed about doing it well. I wrote three scores for that film. The night I finished the second score, I dreamt the beginning of the movie with a third. That was the one we finished with."
He hasn't seen it since. But he still hasn't let it go. "About two weeks ago," he admits, embarrassed, "Jim and I were talking about going back into it, maybe doing a laser disc, putting the songs back. We're still working on it, you know. We should let it go, but we can't. It's like a sore we have to scratch."
Zimmer's next project was The Lion King, which won his Oscar. Now, he's been nominated again, for As Good As It Gets, the first film with Brooks since their shared defeat - a hit, it turns out, with Jack Nicholson as a misanthrope in a twisted romance. "It's the antithesis of a Hollywood score, and I'm really glad that people have noticed it," he says with rare, real humility. "I wanted it to be this subtle thing."
The film had begun in a New Year's Eve power cut, Zimmer and Brooks trading ideas round a piano. It ended the night Zimmer's wife gave birth. He sees each film as a personal journey, the score as a side-effect. As he wrote As Good As It Gets, he had Nicholson's damaged character in his head. It's an identification which has sometimes gone too far.
"I had to move out at home when I was doing The Fan [a Tony Scott film about a stalker], because my wife was starting to get a little terrified," he remembers. "I didn't mind. I can't say it was fun. But it was cathartic. Sometimes you just have to write something to get it out of your system."
It's strange talk for a profession most still think of as hacks, whoring from job to job. Zimmer accepts the term happily. But with one proviso. "I am totally a hack. But what jobs! I write music strictly for myself and I use the Jims, and the Tonys and the Ridleys and the Nics to inspire me. This year I'm working with Terence Malick. We spent a year talking about the movie before he went out and shot it. It's a hellish commitment, for the director and me. You want to be damn sure it will be worth it."
`As Good As It Gets' is released on 13 March. Hans Zimmer's soundtrack is out now on Columbia.