Dialogue coach

In film production, you can either hand all your actors their scripts the night before a scene is filmed and ask them to memorize what they can, and then when they arrive for filming you can have somebody already quite familiar with their lines remind them of their lines while the camera is still running, which in the long run can save a whole lot of money for the producer to use later in other important parts of the production...

Or you can have your actors meet together a number of times off set to rehearse their lines together, which winds up costing a whole lot more, but for some productions (for example The Lord of the Ring), the extra effort pays off a whole lot more in the even longer run...

Doing things the first way is called having a dialogue coach. It's usually, but not always, a sign of either a low budget production or an inexperienced cast or both, that a dialogue coach is named in the credits.