Updated: Aug 8, 2022
A well-rounded character has both good and bad traits, much like we do. A character doesn't (and shouldn't) have to be 100% likeable, either, but readers must be able to empathise with them in some way. Characters, and characterisation, are the threads that bind a story and if we can see a little of ourselves in some of them then that character is successful. Yep, even the bad guys!
When I'm trying to find a character's unique voice, I always use their surroundings to influence how they would speak and act. I try to consider the time period, the social background, even the genre I'm writing—all of these things will (and should) effect voice. Saying that, I also think we often worry too much about finding a voice before we've even started, when all we really need to do is start writing so we can uncover it along the way.
Of course, it's always nice if a character comes along with a strong voice already, but you'll notice that their voice will still change and develop as the story goes on.
And if they still refuse to cooperate, you can always throw them into random situations that take place outside your main story. Write some drabbles or flash pieces and toss your characters into a crisis, or bring someone in from their past and make them deal with it. As their actions and decisions take place, you should get to know them better and it can help figure out what makes them tick.
Interviews and character questionnaires are also good for character-building, though I tend to be careful with these as they can lead to a bit of procrastination. You can find some examples here and there's a more detailed questionnaire here.
Also, the naming process often does my head in and sucks up hours of time. But! Three great resources I've used in the past for finding names (and name origins and meanings) are Behind the Name, Baby Names, and The Surname Database. I nerd out when a character's name has a hidden meaning.
Research is good, but ultimately I find the best way of developing my characters is to just write them—in their own stories, in side-stories, and you can even shove them into other people's stories.
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?” — the latter's voice and attitude 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.
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 another 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.