When I received my first editorial letter from my editor at Scholastic, the biggest change she suggested was a switch from third person limited to first person POV. At first I objected, in part because I had never written a book in first person before. As time went on I came to realize the wisdom in my editor's words (to utilize a good cliche). First person would allow the reader to connect with my protagonist, Satya, in a more intimate manner, which felt paramount for this particular character (perhaps even more so than with your average protagonist...it's difficult to explain).
During my first major rewrite I addressed larger plot and structural issues along with the POV shift, so I basically just changed all the "she's" and "Satya's" to "I's." Of course, there's so, so much more involved with a POV shift than such technicalities, but I just didn't have the mental energy to focus on so many things at once in a single big revision. Since that initial rewrite I've revised the book five more times, and I've come to realize that honing your character's voice is an ongoing process.
I started out with very generic prose. With each revision, I discovered a little bit more about my main character. Her voice evolved. As a writer, I don't think I'll ever be able to fully nail my main character's voice in the first draft. It's something I have to uncover, piece by piece, as I'm writing, and I think it's the process of discovery that makes it so exciting. Sometime during draft #5, I remember reading a paragraph I'd just written and thinking, that's it. That's Satya. When I went back through for the next round, I modeled Satya's entire first person narrative off that one paragraph. Sometimes it's as simple as changing a single word. Sometimes I have to rewrite entire scenes. Most of these changes (at least in my writing) tend to focus on interiority, or paragraphs that directly convey Satya's thoughts. It's not just what she thinks, it's how she thinks it.
I believe it's important to remember, especially when writing a first draft, that it's okay if you don't feel like you "know" your characters as well as you should. Sometimes, it takes a few rewrites to figure out exactly who they are, how their minds works, and the best way to write them. It's also important to remain open to such discoveries. In other words, don't get too attached to keeping your main character exactly the way you first envisioned them.
Writing is a process of evolution, and a character's voice is no exception.