Good directors can create great experiences, while poor directors can create unworkable situations. Actors are looking for directors to protect them on the set. By this I mean that they want the set to be a place where they are able to transform themselves into the character they are supposed to be, without any reservations or distractions. Actors become dependent on directors and vise versa. This means that the director must sometimes step in and inform the actor if their emotional line is off track or if they are getting out of sequence with the shot.
Many directors end up using the same actor's picture after picture.
Once a good working relationship becomes established it is only natural that the collaboration continues. The director and actor will know each other's idiosyncrasies, their style of filming and their approach to a picture. Once they have worked together and established a good working relationship, it is much easier to collaborate together again rather than moving on to someone new. A perfect example of this is the relationship between director Martin Scorsese and actor Robert DeNiro. To date the two have worked on eight different pictures together.