A character's life is lived better by a better version of themselves
A Different Man:
Edward is a struggling actor who suffers from a rare medical condition called neurofibromatosis that causes benign tumors to grow on his skin and facial tissue. This condition has an extreme effect on Edward's psyche where he has no confidence in anything he does.
After undergoing a medical treatment that miraculously cures him, things start looking up for Edward until he meets Oswald. Oswald suffers from the same condition that Edward did, except he effortlessly charms everyone he meets, and is successful at everything he does to the point where he's clearly a much better version of Edward with or without neurofibromatosis. Edward isn't able to handle this and goes into a rapid decline while Oswald takes Edward's role in his play, as well as the affection of Edward's romantic interest, all while doing it with grace.
Synecdoche, New York:
Theater director Caden writes a hyper-realistic autobiographical play, and is looking to cast himself. Someone named Sammy shows up saying he's not an actor but has observed Caden for years and knows him better than Caden knows himself. Caden watches Sammy not just play him, but live his life and understand his inner workings better than he does himself, and embody a version of himself that Caden always wanted to be.