What techniques could a writer use to make the characters sound more distinctive?
One of the problems we writers face is having convincingly distinctive character voices. Writing Fiction puts many demands on our brain. Between story structure, plot, setting detail, action and everything else, something is bound to get overlooked during the first draft. As a result, sometimes the characters in our story, although they are distinctively different people, all come across as sounding the same – like the writer.
Let’s say we already have the first draft, and during our edit we want to focus on character voice. Any advice for how to go about this?
Samuel Loveland 
*pondering*
Character voice can make or break your book. It can change your whole story, because your character’s identities automatically emerge from their voices. And your story happens because of choices that your characters make regarding every stimulus. This is such a critical issue that it needs to come naturally in the very first draft. This assumes that you have created a complete backstory for each of your characters. Try “interviewing them”. This will develop their voice faster than anything. They will be a whole person to you. I do many many drafts until I get the voice and character in my head on the page. If you have already written the character, don’t be afraid of change. You can save your old draft. It will be safe. But I promise that when you write “from character” your book will change for the better, and may even take you somewhere you didn’t know you were going.
Interviewing is a good idea. Similarly, I like the idea of “casting”. A friend of mine does this whenever he conceptualizes his books.
This mostly works if you’re a movie buff, I suppose, but casting actors that you’ve seen in several works (so that you don’t just see them as one character), perhaps even having a picture of them for reference to get your mind thinking about the character’s voice.