During production, Letterman made adjustments in many scenes thanks to the improvisational skills of the actors. For example, Will Smith riffed on movie catchphrases such as "You can't handle the truth" and "You had me at hello" that were added to a scene where he pretends to slay vegetarian shark and new pal Lenny (Jack Black).
While voiceovers are typically recorded solo, Smith and Black did some sessions together in Los Angeles, and De Niro and Scorsese recorded together in New York. "They prefer doing it that way. They can listen to each other and ad lib off each other," notes Letterman. "We used a lot of that stuff."
These recording sessions, which numbered nine or 10 per actor spread out over two years or so as their schedules permitted, paralleled the ever-evolving design and animation process. "As production goes on and we get more used to the actors and watch the tapes of the recording sessions, we become more and more aware of their different ways of moving and gestures and mannerisms; we get more used to the actors, and the animation comes more naturally," notes Lignini. For example, "We tried to match Sykes' gestures to the speed of Martin Scorsese's speech."
One minor but memorable character was elevated from cameo to recurring role. Crazy Joe the hermit crab was initially just in the opening sequence of the movie. "It was a character that storyboard artist Dave Smith did, and he also voiced the character," Letterman explains. "We loved it, and we added the character to more and more scenes."