Try placing your character in a completely different setting. If you really have a grasp on their personality, you’ll be able to successfully transfer them into a new environment. Reimagine your high fantasy character as a modern high school student. Put your contemporary MG hero in a comic book and think about what type of superpower they’d have. Rewrite your period romance character in a science fiction environment.
When you change the setting for your character, you may be forced to consult aspects of their personality you might not have to otherwise. Asking “What would this character study in college?” forces you to consider the character’s interests and skills. Probably this has come up in your original setting. If not it probably should have. This exercise is a good way to check that you’ve figured it out.
If Hamlet was a college student, he’d major in philosophy. For Halloween he’d dress up as a skeleton.
If Emma Woodhouse was on a starship, she’d be in charge of communications. Half the crew would be in love with her.
If Don Quixote was in a horror novel… I’m actually not sure on this one, but I’m pretty sure it would be simultaneously hilarious and terrifying.