A, the protagonist of the novel Every Day, lives every day in the body of a new person. When A “possesses” someone for the day, he (or "she" when A is a girl) is careful to act “in character.” That is, A tries to act as his person would normally act.
That’s the law I’ve set down for myself—don’t disrupt the life you’re living in. Leave it as close to the same as you can.
He tries to follow the trajectory of the day that his person would normally follow. However, there are times when he feels guilty. For example, when he tried to ski for the first time, he broke his kid’s leg and felt guilty the next day because he knew the kid would be suffering for his own mistake.
But when A meets Rhiannon, suddenly A is no longer so careful. Sometimes this results in guilt but not always. He is tired of feeling anonymous and confined, and he no longer acts only as his person would. He wants to act on his own desires. He pursues Rhiannon, and in the midst of doing so, he sometimes gets his people into trouble. For example, when he is the very disciplined and conscientious Nathan Daldry, A goes to a party where Rhiannon will be even though Nathan would never normally go to a party (he prefers studying). This causes trouble throughout the rest of the book. Because Nathan finds out he was possessed by A, he is determined to find A, who he assumes is the devil. While this scares A, it also causes A to feel guilty over his actions. It is his fault that Nathan is in this mess.
But that does not stop A. He continues finding ways to see Rhiannon, even if it means getting the various people in more and more trouble. For example, he is supposed to be studying at the library, but he leaves to go on a lunch date with Rhiannon. The mother finds out, and “George” (A) is in big trouble. But in this case, A does not feel as guilty putting George in this situation, because he feels like he is encouraging George to leave his house. Sometimes the opposite is the case. A sometimes feels guilty not from what he does but from what he doesn’t do. He feels he does not do enough to change his person’s life when he or she has huge problems, such as when his person is suicidal. But usually A knows that there is not much he can do to change a person’s life in only 24 hours. He simply waits it out.
When A wakes up as Rhiannon, however, A is extra careful not to impose on her life at all. He goes through the day as she would—not even wanting to disturb the bookmarks in her book. He feels guilty over his ability to take advantage of the situation: he chooses not to take a shower so he cannot get a glimpse of her body.
As A spends his days doing what he wants to do (pursuing Rhiannon), he realizes this cannot continue. He knows when he is distracted by her, it is noticed by people around him (which at one point almost leads to a breakup with a boyfriend). When A is sad about Rhiannon, he often stays home sick. Again, he knows this behavior cannot continue. His guilt over his actions deepens.
At the end of the novel when A discovers just how much damage he could do, his sense of morality overcomes his desire. He discovers that it is possible to possess a body for more than one day. This is a huge temptation because if he did so, he could stay and have a relationship with Rhiannon. But his sense of guilt prevents him from taking over someone’s life, which he realizes would be murder. Instead, he chooses to run away from the temptation, leaving Rhiannon forever.