Complex sentence: Although "Casablanca" was an A-list film with established stars and first-rate writers, no one involved with its production expected it to be anything out of the ordinary, since it was just one of hundreds of pictures produced by Hollywood every year.
Split into simpler sentences: Although "Casablanca" was an A-list film with established stars and first-rate writers, no one involved with its production expected it to be anything out of the ordinary. It was just one of hundreds of pictures produced by Hollywood every year.