I've always loved Christopher Guest's films - A Mighty Wind is my favorite! He and his writing partner, the great Eugene Levy, supposedly write backgrounds for each of the characters and notecards for each specific scene, outlining the plot, and then leave it up to the actors to improvise the dialogue.
"When I meet actors, I talk with them and get a sense... of someone who can listen and has some kind of a spark." Christopher Guest to Charlie Rose
The improv in his films, "as opposed to what you see in a club," is based on "more real-time behavior," and that's what I love: the heart beneath the humor.
A few years ago, I got to improvise a short film called, "Laundry," and it was a blast. We filmed late at night in an East Village laundromat. My character, Annie, was the proprietor, and the film was completely improvised. The director , Jules Suo, was clear on the characters and types of scenes she wanted. We the actors took it from there!
Below is a small taste of this short film, which is also on IMDB.