I would make a huge distinction between theater improvisation and film improvisation. There isn't much improvisation in film - there's virtually none. The people that theoretically could be good at this in a theater situation don't necessarily do this in a film in a way that will work, because it's much broader on a stage. But in a movie, it has to be real, and the characters have to look entirely real because it's being done as a faux documentary, so there are even fewer actors that can do that on film.
