Even in comedies, you've got to feel safe for things to just happen in a way that is natural and free, and recognizable as human.
My natural accent is American. I chose to speak with a U.K. accent when I was about to enter the final year at drama school in London. I was going to try to find a way to stay in the U.K. after I finished college and could not imagine trying to live and get work there with an American accent.