Practice, practice, practice!
Rehearse scenes before shooting. If possible, at least a day ahead to give people a chance to work out the kinks, and if not, give them a few hours break between rehearsal and shooting. Actors need to work on their roles, sleep on ’em, and dream a bit. It’s usually worth the effort.
We rehearsed all scenes from The Statement of Randolph Carter and I think it helped tremendously.
Although there was still one line that seemed to take, like, ninety-seven takes. I forget the line…
That said, I think I’d like to try a short where we set out with a goal, and then ask the actors to improv the whole scene. Might be an interesting exercise…