As someone who has worked with actors on stage and in film for more than twenty years and has acted himself I can tell you that improv is much more difficult to do well than working from a script. With inexperienced performers, you're likely just going to burn tape on a lot of unfocused rambling and nervous chatter. Or, like Steve said, a bunch of aimless profanity, depending on the age of the performer.
Even many experienced actors are intimidated by improv. There are a few big name directors who specialize in improvised films, but they usually rehearse for many weeks beforehand. That sounds contradictory, I know -- rehearsing improv? But what they do is distill a "script" from the rehearsal, rather than just rolling film and letting the actors ramble.
As Bez suggested, I'd start slowly with improv "exercises" to get them used to focusing on one simple idea, then gradually expand until they reach a point where they are comfortable and are able to stay on topic.
P.S. -- The other side of this is to use non-actors and just have them play themselves doing what they would normally be doing in real life. In other words, if the scene calls for a teacher, cast a real teacher; if you need a shopkeeper, cast a real one; etc.