Here’s a valuable tip for building your story. You have a guy, in a plot, and he sleeps with his friend’s girlfriend.
That is legit story. Things will happen because of that. That will turn into other things. Conflict, confrontation, crisis. Yay!
What is not particularly important is that he did it because in fifth grade, a girl got one of them in trouble when it was the other one’s fault, even though someone else was mistaken about who did it, and maybe it was about stealing lunch money, and there was a girl the first one had a crush on, and she threw him under the bus, and it’s been this rankling thing in their friendship for ten years.
No one cares. It’s too complicated, and it doesn’t remotely justify what happens now.
He slept with his friend’s girlfriend because he’s a selfish jerk.
You don’t need endless amounts of convoluted explanations to set up why people do things. People generally do things because they feel like it.
You will often get a note on your script like, “Why would he do this?” and you have a whole long backstory answer. But no. He does it because he’s a selfish jerk and that’s what selfish jerks do.
This post is so you know you don’t have to develop complex reasons for people to do things to make plot happen, you dig in and give them a personality that dictates they will do this thing.
Then you don’t have to match every single motivation with an ever-expanding encyclopedia of who did what to whom in the backstory, you just have to line it up with the fact that your guy is a selfish jerk. The last thing you need to do is add more story to justify your characters.