Some shows, like PPG, tape in a group session, which is always more fun because you can play off each other.
Start as early as you can. Make tapes of your characters.
Don't be afraid to look silly.
In general, I find that for videos the acting is more realistic.
If I am not right for something there are many talented actors out there that will get it.
I didn't record any additional dialogue for this CD, they are excerpts pulled from existing episodes.