1. One character watching another use the toilet, but not being allowed to use it themselves.
2. A character leaving the bathroom while still desperate.
3. Being made to "Practice" sitting on the potty while diapered or fully clothed. Especially if it includes things like practicing wiping the outside of their messy diaper afterwards, or being praised and allowed to put a gold star on their potty chart in that state.
4. Similarly, a character who can't remove their clothes going through the motions of using the toilet or even squatting down to pee outdoors without actually being able relieve themselves. It could be voluntary (a dumb little girl sitting on the toilet, thinking it might make her feel less desperate) or forced (teasing a desperate girl by telling her to show you how she'd squat down and pee on the ground if she wasn't wearing locking overalls).
5. Being able to tell a character is reaching their limit because their body instinctively starts trying to squat down, or they start sticking their butt out a bit more.
6. Using the fact that a character is diapered as an excuse to not even give them the small dignity of being able to stay out of sight while pottying themselves. The adult sees them squatting down in their room, about to poop, and grabs their hand, "come on. You'll be doing it in your diaper either way, so you might as well come and play with your friends in the living room instead of hiding here. No, I don't care if it's about to come out."
7. There are two girls in the same class at school, and they're best friends. One of them is being diaper punished, and her parents have come up with the idea of entrusting her more mature, responsible friend to enforce the punishment while at school. The first time she needs to go to the bathroom at school the diapered girl immediately asks her friend to give her the key to her locking diaper cover, expecting no resistance at all. That moment when she realizes that her friend intends to fully enforce the punishment would be perfect. Especially if the friend takes the opportunity to make it absolutely clear that she sees their relationship as more like babysitter and naughty child than that of two classmates.