This is a follow-up post to Developing Characters, Part I.
I'd like to go into more depth on character voice and how to develop it. I can only speak for the way I do it; some of these methods might not work for others.
Years ago, when I was trying to strengthen my character voices, I would watch/read/listen to stories that had solid, distinctive characters and take note of the rhythms and phrases those characters used—not only in their speech, but also in their internal monologues. There are subtle differences between "Please make me a cup of tea" and "So are you gonna make me some tea, or am I gonna have to do it myself?" — the latter is wordier, yes, but the voice is more distinctive. Voice (dialogue and thoughts) doesn't need to be spectacular in early drafts, but how characters talk and think should be considered at some stage during editing.
Here's a trick to see how distinctive characters are: take a scene or chapter where there's interaction, and select only the dialogue. Paste the dialogue into a fresh document without any speech tags, names, or other identifying descriptions. Read through it, or have someone else read it, and check where the voices merge or sound too similar. Is there confusion as to who's speaking? If yes, either try loosening up one of the voices or make it more formal. Give one of the characters a verbal tick (like the tendency to say 'you know?' at the end of some of their sentences), or a light accent (though use accent carefully) and then re-read it. Better? It should be.
Another way of tempting out voice is figuring out how your characters feel about what's happening. This will inform their attitudes and moods, and consequently what they're saying and how they say it. Have them react, let them feel, give them passion and the ability to speak up. Let them tell lies. We all do it. An angry character might speak faster in short, snappy sentences, and they might swear or exaggerate, whereas someone speaking calmly and formally might use longer, more complex sentences and have a precise thought process.
Listen to conversations on the street, in your workplace and at home. I've mined real people I know for turns of phrase and verbal ticks. But also remember to look for people who are more guarded, who speak neutrally and try to maintain status quo—often you can have fun with a conflicting internal dialogue and thought.
I mentioned in the previous post shoving characters out of their comfort zones, and this also goes for shoving them at other characters. Bring in the type of person they despise, or someone they're intimidated by, or someone they're attracted to, and see how it changes what they say and how they speak. Character/character interaction drives plot and gives scenes energy. If everybody gets along all the time, dialogue can become lifeless. Even if your characters are friends, have them disagree regularly, or give them a rivalry that you can mine for little tensions.
IMO, most importantly, you'll only get to know your characters well by writing them. Outline and do questionnaires and mind-map them, too, but they have to act and react to really shine.