If you work on network TV, they want to pick your casts, dictate your story and how your show works. It just becomes creativity by committee, which is never good.
Quiet people, people who arent given to emotional outbursts, people who are economic with words - theyre also fun to play, but you find yourself needing a laser precision in those roles. Otherwise you just sort of stand around, looking slightly brain-dead. You worry about being uninteresting.