It only takes one person to make a movie bad, but it takes heaps to make it really good. That’s why when you find something, or rather someone, that works, you don't let them go. Familiarity on- and off-screen is hugely beneficial for actors and directors, as they don't need to re-establish trust and comfort, and they can then challenge one another to grow and change together.
Check out this gallery to see the directors who kept casting their favorite actors, and the actors whose careers are inextricably tied to their directors.