I love my MC. I feel like he has potential to give my story everything it needs. My problem is I tend to know exactly what kind of things he'd do in a big and serious situation, but not so much in small meaningless ones. Like how his day would go if he was doing nothing at all or how normal conversations with other characters would go for him. This is partly because he tends to be a melancholic person anyway, but I still feel like he needs more developing in that area. Any advice or tips?
Write prompt fiction.
I know that sounds kind of strange, but believe me, prompt drabble writing is incredibly underrated. Prompts are a great way to round out your character outside of your story, so when you return to the story itself, you have a better feel for how that character would react in certain situations.
I honestly suggest going on Prompts and Pointers, or checking some of the prompts from WriteWorld. [There are tons of good prompt sites, but these are the two I use.]