Check root meanings.
It’s better to call a character Caleb, which means “faithful” or “faithful dog,” than to overkill it by naming him Loyal or Goodman—unless you want that for comic/ironic purposes. Some readers will know the name’s root meaning, but those who don’t might sense it.
Get your era right.
If you need a name for an 18-year-old shopgirl in a corset store in 1930s Atlanta, you know enough not to choose Sierra or Courtney, unless such an unusual name is part of your story. Browse for names in the era you’re writing. A Depression-era shopgirl who needs a quick name could go by Myrtle or Jane; it will feel right to the reader. Small public libraries will often have decades’ worth of local high school yearbooks on the shelves. Those things are gold for finding name combinations from the proper era.
Speak them out loud.
Your novel might become an audiobook or an e-book with text-to-speech enabled. A perfectly good name on paper, such as Adam Messina, may sound unclear aloud: Adam Essina? Adah Messina?
Manage your crew appropriately.
Distinguish your large cast of characters by using different first initials, of course, and vary your number of syllables and places of emphasis. Grace Metalious (a great name right there) demonstrates this in her blockbuster Peyton Place, as do any of the successful epic writers like James Michener and Larry McMurtry.
Use alliterative initials.
Employ this strategy to call special attention to a character: Daniel Deronda, Bilbo Baggins, Ratso Rizzo, Severus Snape.
Think it through.
You might notice that in most crime fiction the murderer rarely has a middle name or initial. Why? Because the more you explicate the name, the more likely there’s a real person out there with it. And reading your story they might become upset and try to sue you or come after you some night with a bayonet.
Check ’em again.
An author states: “When writing my novel The Actress, I needed a name for a Japanese-American criminal defense attorney, and the name Gary Kwan burst upon me. I loved the name and used it in the book. Only thing was, as soon as the thousands of copies of hardcovers were printed and shipped to stores, I heard from a reader who pointed out the simple fact that Kwan is a Chinese surname. I cursed loudly and decided: a) that I would ALWAYS check name origins, and b) that Gary Kwan had a Chinese grandfather who adopted a Japanese orphan who became Gary’s father. Or something like that.”
Different Actions, Not Looks
Make their actions different and unique. Capture their personality with what they do and not how they dress themselves.
Faults and Fears
A character lacking any faults and fears makes for an unrealistic and boring character. Make them interesting and unique by making them human with faults and fears.
Give them a Part of the World
Ensure the character has someplace to feel comfortable in or someone to feel comfortable around. A part of the world that is their own and nobody else's.
Give them a Unique Goal
In order to make your character unique, simply make them take a side. Give them a goal other than the main plot.
Give them a Unique Speech
Either your character should use unique phrases particular to them or make use of a unique way of speaking. Doing so will help separate your character from any other.
Grow Your Character
Your character needs to develop. not change. Ensure that they learn lessons in order to change their perspective or values. Do this through unique conflicts and scenes.
Values and Choices
Ensure your character has particular values different from the other characters. Once you have that, present them with unique choices that re-enforce or degrade these values.
Who Are Their VIPs?
Who are the most important people in their life: the good, the bad, and the horrifying?
What Are Their Dreams?
Big dreams, small dreams: what do they wish for?
What Do They Regret?
Would they go back and change things, or leave them as they are?
How Do They See Themselves?
Is it different from how they're seen by their spouse? Their boss? Their friends?
Which Locations Are Most Important To Them?
Is there somewhere they're desperate to travel to? A place they'll never forget?
What's Their Relationship With Food?
Do they scarf it down or savour every bite? Cook a special dinner or order takeout from a restaurant?
How Are They With Money?
Have they spent every penny they've ever earned? Are they resting on their laurel? Investing in stock and bonds?
Are They More Emotional or Logical?
Do they believe in concepts like fate or serendipity? How are they most likely to resolve an argument?
Are They Spiritual?
Do they have a religion? Are they tolerant of other beliefs? Have they ever used their spirituality to justify something abhorrent or judgmental?
Spitefulness
Pettiness
Cowardice
Dishonesty
Cruelty
Self-denial
Naivety
Gullibility
Messiness
Insincerity
Arrogance
Vengefulness
Delusion
Rage
Self-pity
Selfishness
Hypocrisy
Laziness
Entitlement
Prejudice
Defensiveness
Prudishness
Unfairness
Tactlessness
Paranoia