But actors, certainly not high profile ones, are cast for their ability to do a credible accent. They are cast for who they are, and the accent comes later.
If a studio or a director wants, for example, Jack Nicholson for a role, then they want him because he is right for the character and his name will probably help sell the movie.
If he is required to use a different accent, he will spend time with a dialogue coach...he may get the accent bang on, or he may come up with a passable approximation of the accent.
But the bottom line is that people who like Jack Nicholson and want to watch the movie are still going to pay money to see the movie, bad accent and all.