What I find fascinating is that your characters are so different from mine, yet they're very real. Just as you're a very different person from me even though we get along well.
It's not just because they are born from separate heads, though of course that's a lot of it, but because our character's voices are vastly different. When writing from the third person limited or first person point of view, it's vitally important to find the lead character's voice, make it consistent, and make it unique. How exactly do you do that, though?
The first step is to know your character. Since I've been in Caprice's head for a month, I'm going to use her for an example. The questions I ask myself when I'm making a character go from the pretty vague to the very specific. First, what are her major personality traits? Well, she's a sociopath, but what does that mean exactly? It means she doesn't have empathy for other people. That leads to a lot of coldness in her voice, saying, "Yeah, this person is going to die. Oh well." She doesn't care. They aren't important to her, unless she needs something.
What's another trait? Well, she's vain. She knows she's good looking, and she wants to stay that way. It's not because of power or to use people, though she does use her looks to get what she wants, it's because she wants to be the best. Being the best to her means being good looking as well as powerful and smart.
Though there's more to her, those are a couple of broad strokes I can build off of. I can remember that she would be upset when someone rebukes her advances because hey, she's hot. I can put down how little she cares about someone else's pain (except to get off on, but that's another personality trait), and so on. Once I have a good sense of who she is as a person, I focus on what she does for a living. Working is a major part of a person's life. In this case, she's a hired killer or thief. Really, a hired thug for whatever dirty job. Is she picky about it? Are there things she wouldn't do? What would be her line?
If someone were say, an office worker, reasonable questions to ask about them would be if they liked their job, and why? Do they have an office romance or rivalry? Is there a task they don't like doing, but are forced to do anyway? ("Gonna need those TPS reports...") Do they steal from work? So on and so forth. Another thing to ask the character about--and by this point in my head I'm asking the character, not just myself anymore--is their family. Do they get along? Have a big family? Maybe they were an orphan. What family controversies were there, if any? What traditions? Do they miss their family, or do they talk to them?
In Caprice's case, she would answer something like, "I don't talk to my family. They don't have anything to do with who I am. I'm my own person." And she wouldn't say much more until pressed, at which point I knew she was sore about her family, for whatever reason. "I don't like my mother. She pushes, too much, too hard. She doesn't let me do things on my own. But I like my dad. He was just always there, you know? He didn't take sides. He was always there for both of us."
Now is where it gets fun. You have an idea about their life, what they do, and who is around them. Ask yourself this... what do they order for pizza? What's their favorite television show and why? What do they do for fun? Ask them any other questions you might ask someone in getting to know them. Favorite hobbies, interests, how they like their food, anything at all! In Caprice's case, she orders a large everything. She doesn't watch television, because she finds it a waste of time. And her work is fun for her... but talking about something else other than work, she likes people watching.
Getting a handle on their personality is vital to 'hearing' them. So when you get into the nitty gritty of describing their voice, you can tell if it's going to be soft or hard, whispery or loud, and so on. Do they have an accent? Are they from a particular area of the world? How they express themselves to others in the book is how they are going to express themselves to your readers. Are they guarded? Secretive? Open? Trusting? The main character's voice is going to set the whole tone for the book, and although it's important to hear all of the character's voices, the narrator is the most important.
In this case, Caprice speaks with no discernible accent, but she's confident. She doesn't have to raise her voice to be heard, but she can when she wants to. She's not particularly secretive, nor open. She has secrets that she keeps, but everything is an open book because why should she care? She knows she can take anyone down if she needs to. Her confidence spills over into arrogance, actually, which some might find endearing, and others might find bitchy. She doesn't care what they think, because the only one who matters is herself. Hence, the tone of the book is her finding the limits of her confidence and abilities. While she might get knocked down from time to time, she always comes back stronger. She's also sarcastic at times, which I get from her arrogance. She sneers at people because she views them as weaker than herself.
Might be a rambling post, but it was something I was thinking about anyway. A character's voice is only one part of a book, but it can be a major part. It's not even going into the MC being an unreliable narrator, which may lie directly to the audience. I may ramble on about that in a different post.