I lose track of what's going on in a movie because I get distracted with overanalyzing the background characters' acting capabilities.
That's an unusual thing to do. Why do you get distracted by it?
I like understanding the art of their performance and how they compose their characters
That's an interesting thing to do. You must enjoy it, and you're free to enjoy a watching a movie like that if you want.
It's funny when people do something wrong and it seems like they don't know better.
It sounds like you still really enjoy the movie even though you lose track of the main plot. There's no reason to feel ashamed about finding enjoyment in the small largely unnoticed parts of of a movie.