The English actor, renowned for his "total" approach to a new part, prepared for his leading role as a poor, Irish boxer by repeatedly getting into the ring with the former world featherweight champion Barry McGuigan.
But this was not just the occasional choreographed bit of sparring. Day-Lewis spent months training up to professional standard and, since finishing the film, has continued to indulge in the risky pursuit whenever and wherever he can.
"He was just magnificent," said McGuigan of his work with the star. "He was about 160lbs, so a bit middleweight for me - he would have knocked me out - but some of the professional sparring partners I set up for him produced sessions that were better than the fight scenes in the film. We had to skip past 10 years of amateur training that any other boxer would normally do, but he took to it like a fish to water." It is the kind of whole-hearted commitment to rival Robert De Niro's infamously obsessive regime in preparation for his role in the ring as Jake La Motta for Martin Scorsese's Raging Bull.
Day-Lewis, son of poet and writer Cecil Day-Lewis, already had a strong interest in the sport and was keen not to use a stuntman in the fight scenes that feature in the Jim Sheridan film.
McGuigan recalled: "In the end I was saying to Daniel: 'Look, we can make this easier for you,' but he felt he could never feel what a boxer feels unless he really went through it all. His attitude was exactly like mine. We have become very close."
For the film's co-screenwriter Terry George, who also worked with Day- Lewis on In the Name of the Father, the actor's attitude to the part and to the sport were both predictable.
"It is a whole dimension of Daniel's character," said George. "He is a Spartan and he has a very strong competitive sense."
The Boxer, which is set in Belfast and tells of the damage inflicted on the community by sectarian violence, premiered in New York on Wednesday night and has already been nominated for three Golden Globes - for best actor, best director and best film. It also stars the actress Emily Watson, acclaimed for her performance in Breaking the Waves, and opens in Britain in March.
Arthur Lappin, who produced the film, guesses that for Day-Lewis the appeal of the part was the opportunity it provided to conquer his physical fears.
"In spite of a very painful back, caused by a herniated disc, that was obviously giving him pain during the premiere, I am convinced Daniel will carry on boxing until he drops. He loves it."
The film was inspired by the life of McGuigan himself but, the boxer- turned-commentator says,his own life was "just too clean".
"I felt a real onus to make the fight scenes look right," says McGuigan. "In the end, Daniel had a left hook that Sugar Ray Leonard would have been proud of ... He had black eyes and cuts, but didn't worry too much about health insurance and things like that."Reuse content