Let your characters talk to each other and do things. Spend time with them - they'll tell you who they are and what they're up to.
There are a million little things, but one of the best ways to get to know characters is to just put them in situations and see what they say.
I think as an actress, I prefer having a character on the page. It allows you to be more invested in actually creating a whole person. It's easier when you're not trying to come up with your next line on the spot.
Sometime female characters, especially in the genre of something that people consider rom-com, make mistakes in a cute way or they're a mess in a way that's palatable. I like that.
