I write a lot about characters and characterization. It's probably because the characters are such an important part of the coffee house book (and the other books in the collection). In fact, the characters tell the stories. The characters are the stories.
When characters are that important to a story, you have to understand them. You have to know everything about your characters, even the stuff that never makes it into the story. You have to be in your characters' heads in a way only a writer can be. But it's more than simply knowing your characters because you created them. You kind of have to stalk them. And there are writerly ways to do it.
Extensive character biographies
I've written on the topic of character biographies in the past. (Here, here, and here, for just a few.) I think they're useful tools for writers in developing your story. Finding a good biography template and using it for your major characters (and sometimes your minor ones*) can help you understand your characters. You can use it to develop and know your characters before you start writing, and you can refer back to the biographies to refresh your memory as you write. (Is Vivi's favorite color teal or orange? Is Colin allergic to macadamia nuts or peanuts?)
When you have a character-driven story, the biographies are even more important. I've mentioned before that I use the life story interview in Atkinson's The Gift of Stories, and I'm very happy with it. Sure, there are aspects of it I skip for different characters (for example, Vivi isn't married, so I skipped the section about marriage), but it gives you a really good foundation for your characters. And, life story interview aside, it's a great resource for story-crafting. (In fact, I'll be ordering myself a new copy soon since mine is just about worn out.)
Put your characters in real situations
Of course, this can apply to your stories, but I'm thinking more in the "writers think about their stories/characters all.the.time" realm. You know what I'm talking about. You have a strange encounter with a cashier or find yourself lost on one-way streets downtown, and you wonder, "What would [CHARACTER NAME] do in this situation?" Answering those types of questions can help you get to know your characters. And the more you answer, the better you know your characters.
After all, your stories aren't going to simply be talking-head stories (I hope), so you're going to need to know how to incorporate your characters into real situations, and have the unfolding conversation/situation read as realistic. Even with willing suspension of disbelief, there have to be facts. Just ask Dana. Because even in the most fantastical story, there are elements of realism. There are people and situations and conflicts that are recognizable to readers. So think about those people and situations and conflicts. Put your characters in them.
Yes, it's a lot of work to get in your characters' heads. But in character-driven stories, it's necessary. Just as you know your plot inside out in a plot-driven story, you have to know your characters inside out in a character-driven story. The better you know them, the stronger your story will be.
How do you get in your characters' heads?
*I have a character I thought was minor who was to be in all the books in the collection in some capacity. After writing his biography, he's turned into a major character. But still doesn't speak for himself.