One of the things that I like about doing theater at Musical Theater Village in Irvine is that most parts are cast at least 4 deep. Actually, that is both the good and bad. If too many people are cast for a part, your shows are limited and if you have 3 or less shows, you barely have the opportunity to polish your part. Myself, I prefer about 8 shows over a 6 week period.
Multiple casting allows each actor the opportunity to watch and learn from the other actors and it seems that no matter how good or bad the other actors are, they always seem to do at least one thing different or better than anyone else and there is always something to learn from them. You might like the way that they cock their head on one of their lines or an expression.