At a recent audition for an indie film, while I was using my prep time, I overheard two other actors discussing the sides. One had decided to give the character a British accent. Why? She wanted to 'make it more interesting.'
I don't know how much research this actor had done, but the emails from the casting director had included a link to a website with more info for the film. From looking at that website, it was a pretty good guess that the characters they were casting at this session were at best, supporting characters, and probably in reality more like under-5's. Characters who were not the focus of the story. Characters who were there so that the leads had someone to talk to while they revealed major plot points, grew and changed emotionally, etc.
Not characters who the viewer of the film was supposed to think 'that character is so interesting - I want to know more about her.'
I'm not saying these characters shouldn't be well-rounded, fully fleshed out, have an inner life. Of course that's true. But it behooves the actor to know why the character is there, what function the character has in the story, and to let that knowledge inform the choices.
Don't be funny if your character is the straight man. Don't take focus when your character's function is to give focus to the lead.
Figure out your job, and do it. If you trust yourself with that, it's gonna be interesting.