Ed Norton's character has some obvious problems. He goes to meetings and sessions for people with diseases or other problems such as testicular cancer, sickle cell, tuberculosis, and others. He goes to these things because he can't sleep due to the fact that he is depressed and needs to let feelings out. All of this is compromised when a girl named Marla Singer joins some of his groups and he can no longer cry or sleep to satisfy his id and ego because knowing that there is another faker present, he feels guilty. All of this is settled when a part of him comes to life when his ego (Brad Pitt) comes to life.
Brad Pitt's character represents Norton's character's ego deep down inside. He is in every way what Norton wishes he could be like. Pitt's character's id was to keep Norton thinking that he was real because then he can lead Norton to success, which leads himself to success. He himself is an ego, and the things that he does that connect with his personality is that he lets things happen, he's kind of nuts, like how he shook his blood in the face of the guy who owns the bar.
Marla is the kind of person that has no conscience, no id present. She walks on in the middle of ongoing traffic, smokes 24/7, casually steals things, and her appearance lacks effort a lot. However, she has quite an interesting ego. She has a careless sort of personality and smokes all the time, she always goes to Norton's house to have sex, and later has feelings for him. Marla can also represent a superego to Norton's character. she is that outside power making him mad and upset and in the end he cares about her a lot.
There are lots of examples of id and ego that are portrayed in the film, and they have a lot of different ways in which they are shown. Pitt's ego and id are probably the most powerful since his main job is to be an ego. Marla is the second most powerful, and they both have one thing in common, and that is that they both control Norton's character.