Improvisers have one thing drilled into them from day one: Yes And... It embodies the entire philosophy of how create something out of nothing. It's great. "Yes and" gets players to agree and cooperate. Players get so caught up in "yes and" that any denials feel wrong. Even criminal.
Jill Bernard tells a story in her workshops about how other people in scenes with her would endow her character as a stripper. Appropriately, she got tired of this, but in order to avoid denying their offer she needed to be a character she was tired of playing.
So she came up with an idea she called "Yes/No." She'd play the stripper, but she'd play a stripper that didn't want to be a stripper. Her stripper characters would dance, but while she danced she'd cry and say she wasn't going to dance.
There are plenty of times in improv where your character needs to do something in order to move a scene forward. Like you might be a steadfastly moral character who would never give into temptation, but that scene where you don't give into temptation might not be that interesting. So pull a yes/no. Go ahead and have your character agree and do something, but have your character not want to do it.
This actually works really well if you've been denied. As part of an exercise, I was supposed to agree while my partner denied everything. I was told I was in prison, so I made friends with a mouse. Then my partner told me, it wasn't a mouse it was a grapefruit. My character knew it was a mouse and that the mouse was my friend. But my character also had an evil prison guard telling him that his mouse was not a mouse. The yes/no gave me an easy way to justify both contradictory offers. I ate the mouse, but was apologizing the whole time to my little mouse friend.
So my friends, consider the yes/no. Have a character do something they clearly don't want to do. Have them say they aren't going to do it the whole time they're doing it. Have your crazy athletic character binge on chocolate cake all while claiming that he's not going to eat this cake. It's fun and different and in some cases can let you have yes and no all at once.