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; 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.