My favourite way of figuring out an OC is to be able to play them against someone or write them - which, if you're scared of them reacting conveniently, would probably be the better option - until I start figuring out more details and intricacies to their personality, apart from just the general stuff. But while doing that - especially if I'm rping - I keep the personality section nearby so I can go "well, what *should* they be doing, within the confines of what I know right now?" I like giving my characters little vocal quirks, personally, or some personality quirk that can be used to further conversation (for example, I have a writer who asks people a lot of questions about themselves to learn more about people, and a police officer who is too nice to admit they don't get sarcasm so they end up inadvertently playing along). Kind of a cheat, but it helps keep the character interacting with people.

Also! A good beta reader is the best thing you could have with an OC app. It's so easy to write things you think make sense that completely don't to an outside party.