For my money, it's arguably the best scene in a movie filled with great scenes.
Brando really had to draw on all his acting chops in that scene with Luca's awkward speech:
According to Harlan Lebo, there was much practical-joking on the set. During the filming of that scene, Lenny Montana, who played Luca, taped a piece of paper on his tongue stating, "F**K YOU" and stuck his tongue out at Brando. I guess Marlon had all he could handle to keep a straight face after that.