"By the time the cameras rolled, the star was an extremely fit and well-trained cockroach. He had worked with La Motta for nearly a year, boxing some 1,000 rounds in a gym on 14th Street, adding 20 pounds of muscle to his 145-pound frame. He learned La Motta’s crab-like style, and inflicted some damage on his mentor (chipped teeth, black eyes, a broken rib). De Niro had seen too many boxing pictures where the actors didn’t look like real fighters and he was determined to exhibit the true hardness of the trade. Indeed, he got so good that La Motta scheduled three professional bouts for him (anonymously), and De Niro won two of them.
When the work in L.A. was completed, the actor embarked on an eating binge in Europe, during which hiatus the production moved back to New York, awaiting his return. It took him four months of grand European dining to gain the 60 pounds required to play Jake in his decline. He had made all of 1900 in Italy, and a portion of The Godfather, Part II, and knew all of the best pasta restaurants. The actor says it is as hard to gain that kind of weight in a hurry as it is to lose it, and when he returned, Scorsese was worried about De Niro’s health, especially his labored breathing. Of course this devotion to the cause of realism was all the press wanted to talk about when the film was in pre-release. The art of acting, for all the words expended upon it, remains a mystery to the public. But something like weight gain—any sort of physical contortions an actor submits to for his art—that’s something the public can relate to with headshaking amazement."