It's great when improv is encouraged. It's a really fun thing. It depends on who's in the movie and how their process works, as well. It takes a director who is open to that because you have a script, but then something funny could happen on set. So, to have people around you who encourage improv is really exciting.
Teachers believe they have a gift for giving; it drives them with the same irrepressible drive that drives others to create a work of art or a market or a building.