This is something I've been thinking a lot about lately. Voice is very important to writing. You (as a writer), must find your own unique writing style and yet must also make each character stand out and not let their voices blur together. Real-life individuals don't all sound alike, speak alike, or use the exact same words. Dialects differ. Idioms change from region to region. Just as in real life, so it must be in your novel.
It's all too easy to fall into the trap of zinging through your writing, not thinking about anything but getting words on the page, and that's not necessarily a bad thing. Where it gets dangerous is when your characters begin to all speak in the same tone, have the same mannerisms, and react in the same predictable way.
Donald Maass has an excellent book called Writing the Breakout Novel Workbook. In it, he devotes an entire section to voice that is really helpful in figuring out your characters' individualities, their own personal idiosyncrasies, etc. One exercise was to take a specific word and think of several different ways to say it. I.e., bureau, dresser, or chest of drawers. This was very helpful to me in "hearing" each character's voice in my head as I wrote their dialogue.
Another thing that will help in making your characters unique is to write backstory on them. Don't put it in your novel; that will clog things up unless it specifically furthers the plot. But sitting down and concentrating on each of the main stars of your book in turn, creating family, background and history for them, is hugely helpful (at least to me) in defining that character within the novel. Once I know where they're coming from, I know where they're going.
It may feel like a waste of time to you to write stuff that you know won't actually be going in the book, but I promise you it is not. Your characters will be more richly developed, complex and unique unto themselves, which will make your readers all the more interested in learning more about them, which will assist in the turning of pages and buying of your books.