One of the things you will be doing is listening to cartoons, anime and animated features. There is a grid you will need to fill out for that, much like you did for the Radio and TV spots. You will start to hear that some animation (including video games) doesn’t have extreme goofy characters, but real sounding, fully fleshed out characters with their own personalities.
Sometimes if a star is doing the role, there is no doubt who it is, because they always sound pretty much the same way. Tom Hanks and Ray Romano come to mind. But that is probably why they were hired!
On the other hand, think about John Leguizamo, who played Sid the Sloth from Ice Age. Here is a clip where he explains how he came up with the voice.