Lost
Lost is a long form game for six or seven players. It's based on the format of the TV show Lost.
Actors begin a scene set in a location the audience suggests. Once most of the characters have appeared on-stage at least once, the ref (or, sometimes, an actor) freezes the action and asks the audience, "What is this person's secret?" The audience suggests a secret, which is sometimes relevant to the scene (e.g. "All the money's gone") but is often just like a party quirk (e.g. "She's scared of food.") The ref then says "Let's say how that happened" or "Let's see a little more about that" and the actors perform a brief scene explaining the secret. Afterward, action returns to the main scene.
Eventually, the audience will suggest secrets for each actor. Once we assign a secret, an actor should change their behavior accordingly, unless the secret justified some character element they'd already expressed. The flashback scenes may incorporate characters from the main scene, somehow tying all characters' pasts.
The action should build toward a climax that brings plays all the characters' situations against one another. Characters may or may not reveal their secrets to the others.