I have made one improvisational short film before, and if you're not careful, it could end up on the next video casette in the garbage.
We came up with a story idea, and from that point on I put my faith in the people on screen. Luckily, they wre funny people, so it worked out halfway decent.
As Harold Ramis once said, improv is a comedy tool and it is always planned.
It doesn't sound like it makes sense, but when you look at improv scenes, such as the scene where Ty Webb and Carl Spackler meet in the Greenkeeper's house in Caddyshack, you can tell that it was not only improv, but it was planned.