Hi LDK_SPARDA,
Good question. Usually for me it is just looking at the character imagining what voice would come out. If you saw the Hey Look! Short I did for Oh Yeah Cartoons and Frederator, I wanted the characters to have that 1940s New York average Joe feel to them, and Maurice LaMarche, and (Oh my god, I can't remember the actor who played the big guy. That is so embarrassing, because I've dug his work on live action stuff for years before I used him on that. Oh egg on face.) but they both fit that bill perfectly, I think knocked out of the park.
On Pete Patrick Private Investigator Danny Mann came out with what I thought was the perfect voice for Doctor Don't. He was evil and perfectly happy about being so. Pam Segal I thought was just right for Pete, and I knew even as I was drawing it that I wanted Billy West to do his Peter Lorre as the fez wearing Persian Puss. We had to cut it for time, but I had Persian Puss hitting on all the ladies they ran across.