Casting will generally take longer. Finding the right actor for each role can take longer thab the right key people for yourr crew.
There are always "Personalities" on set.
A crew person's demo reel says a lot about their work. With actors, too many don't have demo reels and the ones that do may not have demo reels that really reflect what they can do.
Depending on what type of production you are shooting, you may also need to rehearse actors for say an action film. Actors tend to ignore posted schedules. They like to make up their own schedules. Those you don't need. They also try to get you to change a shooting schedule and sometimes an audition just for them. Crew people, for the most part, are not that disorganized. It takes time to filter them out.
Megan Fox got fired for trying that stunt with the last Transformers movie. Steven Speilburg fired her for trying to set her own hours and spending too much time on her cell phone.