"Nothing is worse than spending countless hours animating a character's mouth to find out that what he says doesn't really work," said producer and audio engineer Bryant Falk in explaining why the voices come first when making an animated film.

Part of this is about making sure the characters seem as lifelike as possible when they're speaking, but part of it is also about capturing the spontaneity of the actors.

"The great thing about voice-acting is it isn't animated yet," said Alan Tudyk, a live-action actor ("Dodgeball: A True Underdog Story" 2004, "A Knight's Tale," 2001) who has lately started taking more voice roles. "And so once you find the role and you're within the scene, if you come up with an idea, you can go with it very easily. It's all in your head."

That's not to say it's all off the cuff. Actors, of course, are given a script and are expected to stick with it, like any film. And Tudyk admits that most of his improvisations get cut anyway.

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