Most people would list a few other rules before this one considering this idea "advanced" - but I think it's so essential, I'm going to consider it necessity.
Improv Rules 04: Find the Game
The game of a scene is the dynamic interaction between the characters of a scene. It is not necessarily what the scene is about. It is not the plot of the scene or the story of the scene, but it is still what makes the scene work. It can take many forms.
Example:
Jennifer and Ted are cleaning the house, Jennifer finds something on the counter and tosses it into the trash. As Jennifer continues to clean, Ted walks to the trash unseen by Jennifer and removes the item, examines it, then puts it into his pocket. They might be talking about the neighbors or their future or anything - but the game is suddenly that as she finds and throws out things, he will try to rescue them.
The players need to be conscious of this. Jennifer needs to be aware that something worked after she threw out something - even if she didn't see it. So, she should try throwing something else out. Ted needs to be aware of his partner and get that she is feeding him another opportunity and he needs to take it.
This interaction is akin to musicians suddenly improvising a song together and it's very fun for an audience to watch these two practiced performers discovering their chemistry.
Step Twois to Escalate the game. The first time this happens, it's funny. The second time, it's even more funny because the audience is now part of the inside joke. The third time needs to have a twist. Jennifer throws something out and then turns quickly around - something she's never done before - maybe Ted was just leaning down to the trash and is caught - but:
Step Three - don't label the game. Think of this as a suggestion because there may be times where it would work - but in general, you never want to point out the game in the scene. The audience doesn't want Jennifer to say "you were just about to take that thing I just threw away out of the trash!" Then the joke is over. Jennifer needs to allow Ted the chance to succeed in pretending he was doing something else. He picks up the trash can, sweeps a little under it, sets it down. And this gives a chance for the game to continue to grow in the scene and continue to escalate. Now - when the scene is rightfully over, Jennifer can label the game with a closer line like "You might want to check the garbage in the garage - I went through your magazine collection earlier today." (even though we think she didn't realize he was taking stuff out... so the joke works in context.)
So - the game is not the subject of the scene, it's the game between the actors. The first example I provided is non-dialogue based, but the game can include dialogue of course.
another example:
Michael and Tanya are trying on clothes - Michael says "I'm glad you don't look like a super model, I wouldn't want to date a supermodel" which crushes Tanya - but he means it in a positive way and he keeps trying to re-assure her. Michael says, "I don't want to be with a super thin young girl, I want a woman." She cries. "No, honey, i'm a jealous guy. If other men were watching you all the time, thinking how beautiful you were, I'd go nuts." Basically, the game is that Michael WANTS to say nice things, but everything he says makes Tanya feel worse and worse. The subject of the scene is that they are clothes shopping. This is what is happening while they are doing that. Obviously, this example is more central to the focus of the scene, yet it is still the game.
And, the rules still apply - Escalation is necessary to keep it interesting. And neither character should label the game or it's over. If Tanya says, "I know you want to say nice things to me, but you're just making me feel worse." Dead. The game dies an ungraceful death.
The game can be applied in drama as well - it does not always need to be comedic. The game can tell the subliminal story between characters in any genre.