There are only two ways to heighten an improv scene. Only two ways to put it on a clear path that goes inexorably toward a conclusion of increasing importance and excitement.1
You can heighten the situation. When we use the phrase “If This, Than What?” we are usually asking what situational course logically can follow from the pattern we’ve established. If the baker made you an erotic cake for the retirement party, then what other things can he do that are similar but bigger? Boob cupcakes for the boy scout event? Naked man covered in pastries laying motionless on the adjacent table and ready to be rolled out to the Mother Superior’s birthday?
I think when most of us envision heightening this is the pathway we imagine: playing the pattern to its zenith. However, the stakes can also be raised in another way. The situation can stay essentially the same if the reaction to the situation increases.
Emotional heightening is a second pathway to heightening and it caries the advantage that you can essentially decide on it before anything else is established. For this reason I recommend it to my students who have difficulty heightening through pattern: you can get on a stage and decide “everything my scene partner does will make me hornier” and the scene will heighten. You can choose “more upset” or “more delighted” or “more suspicious” and with your reactions in hand the scene will be on a non-stop heightening track regardless of what the situational embellishments or additions are. If the baker has made an erotic cake for the retirement party and you are appreciative, then he explains that the icing is buttermilk and you are thrilled, then he tells you that he has to go on break and smoke behind the cake shop and you are elated, and he says you can’t go back there and that rule makes you ECSTATIC: you’ve had a scene with no premise or situational pattern that heightened none the less. It was controlled completely through reaction.
Reaction is the greater part of improv.
The most inventive, wildest, or cleverest choices can be made in an improv scene by one improviser but if their scene partner refuses to be affected by them the scene will feel hollow and flat. However, innocuous or small choices can be exalted by a strong reaction and can lead to a delightful scene. Therefore, never forget what power there is in simply reacting to what is happening.
Heightening creates clear scenes that feel good and work. It doesn’t have to be a complicated equation: go for the next link in the pattern or, if that somehow eludes you in a given scene, simply respond with the same emotion to every new revelation and increase the intensity of that emotion.