I always read the entire screenplay with all the actors doing there parts before I shoot, and then sometimes we rehearse difficult scenes, but I always try to work with the actors believe it or not some actors would rather not rehease. others thank me profusely.
As far as IMPROV goes it depend on SP, actors, and the scene.
Improv can make actors look more natural, but if the actors are not on the same page it can be disasterous. Not to mention continuity issue in post.