Martay
Veteran
Use psychology. Have better actors overpower them to put them in your place. Like everything else in a film, you must learn to master it and master them, or they will master you.
Other actors are an excellent resource when you're not getting results that you expect or want. I have relied on other actors to push certain actors in order to gain a stronger performance. I think it also helps the actor in question without making them feel like they're completely letting you down. If the other actor forces them to step up, then there's a certain amount of "fun" for the actor who wasn't necessarily delivering the goods just based on the main direction. When they "mix it up" with an actor in the scene itself, it sometimes takes away any of their external thoughts about hitting marks just so, etc. that can often get in the way of good performances.
Of course, this presumes that the other actors are able to do so. Which comes back to casting decisions, etc.
-M