Start as early as you can. Make tapes of your characters.
In general, I find that for videos the acting is more realistic.
Some shows, like PPG, tape in a group session, which is always more fun because you can play off each other.
I usually have 10 different animation projects going at a time.
I think it's definitely beneficial for these characters to have good acting voices behind them and it affects the characters in a way that people can feel like they're part of the game and that they know these characters.
Don't be afraid to look silly.