Sometimes novels grow organically from inspiration, and every character you create feels like a real person who has been living in your brain for decades; they come complete with back story, personality, and a visual.
Sometimes we create characters because the plot requires them for some reason, and they show up to the story as robots, sans personality. Sure, they can pull their plot levers as programmed, but they’re not very fun, and not very interesting. As the writer, it’s up to you to get to know them a little, so you can make them interesting.
If you’re not sure how to do that, steal a page from Marcel Proust.
Or Ratatouille, if You Prefer
If you’re unaware of Proust and his epic masterpiece Remembrance of Things Past (a.k.a., In Search of Lost Time), shame on you. It’s more than 4,000 pages long, so there’s no shame in admitting you haven’t read the whole thing—but you should at least read the “episode of the madeleine,” which is deservedly famous. The short and soulless summary is that the narrator eats a type of cake he hasn’t had in years, and the taste transports him into his past and the specific sensations tied to the experience of eating one.
It’s essentially the same thing that happens to Anton Ego in the film Ratatouille when he takes a bite of the titular dish and has a flashback to his simple, joyous childhood. And you can do something similar with your characters as a way to get to know them.
Start with something appropriate for the basic character as you’ve envisioned them. Would they eat cakes? Cookies? Or is it a slug of whiskey, or a sip of beer that brings them back to the day their father gave them their first taste, or the last time they drank before going into AA? A cigarette that brings them back to their schooldays? It doesn’t matter what you choose, as long as it’s a sensation that evokes a response. Then, build out that memory. Explore it. See what details it reveals to you.
Just be prepared to crave a lot of things after you’re done.