The Scottish comic plays a former opera singer in the comedy, about a retirement home for ageing musicians.
Billy said of Hollywood star Dustin: "He gave us loads and loads of freedom and not only that, we would do these 12, 14 hour days like you do when you're filming, and then he would go off and work at night on the script, then he'd come back in looking a little tired but behaving like 100 per cent, having changed a lot of things.
"It kept the whole thing alive and well. You'd learned one thing and in the morning he'd say, 'Never mind that, you've said that anyway'."
Billy - who acts alongside Dame Maggie Smith, Pauline Collins and Sir Tom Courtenay in the movie - added: "He was tearing pages out of the script saying 'You've said that already, you've said that before'. and he said, 'That's the difference between you and us, the Americans make movies, the British make talkies.'
"I think he's bang on, the Americans will cut to the chase whereas the British, to their great advantage, will verbalise all day, bla de bla de bla!"
:: Quartet is released on January 1.