The tiniest things reveal nuances about your character. These nuances make your character unique. These are the small things that your readers may never, ever in a million years realize you're doing. The simple way your character swears in their head or out loud. Do they say "God" in situations of stress or brilliance? Are they saying it because they were raised in a religious household and their first reaction is to begin praying? Or, the other end of the spectrum, they use "Oh God" as a blasphemous swear because they weren't raised in a religious household? Or does your character use it because she's a teen and that's what teens do (in which case, she's crossing dangerous territory into normal rather than being "normal")? Or are you being a lazy writer and didn't realize that you use it as a filler, empty calories, wasted space that could otherwise be put to great use?
For example, consider the below passage:
The boy before her was the most beautiful hunk of man meat she'd ever lay eyes on. My God, she thought, as her eyes traced his broad shoulders, and settled on those heavy brows over intelligent green eyes.Ignore the man meat--if you can--and the ridiculousness of this passage (felt like writing a bit of romance rather than YA like I normally do for examples. Go with it). Okay, we get that she's a lusty woman and the "Oh God" is as much a physical response as her brain stuttering. But consider:
The boy before her was the most beautiful hunk of man meat she'd ever lay eyes on. Good Cosmo Lords, please save my loins from what I might do. Her eyes traced his broad shoulders, and settled on those heavy brows over intelligent green eyes.And immediately, you know who the woman is, and that she's not just a silent observer--she's a woman of action and Christian Gods have absolutely no place in her life. Or bedroom.
Language shapes your characters, setting, world, absolutely everything about your writing. Consider John Green, any of his works. You won't find a single misused word. Any swear words are carefully thought out and, in the case of An Abundance of Katharines, use Frack instead of F*ck. Because they're nerds and it's awesome.
You notice that I'm talking contemporary novels rather than SciFi, Fantasy, etc. That's because I didn't want to get hung up on the intense world building that goes into those genres. But it needs to be stated. Loudly. Your special worlds need special vocab. It can be simple as substituting a swear like "Oh God" for "Oh Gods" or "Oh Goddesses" or getting really specific and going for "Good Leopold and all names of good that spring forth from your fingers", Leopold being a king or god or creator of the universe. Or heck, an artist the main character admires greatly.
I don't think The Girl of Fire and Thorns by Rae Carson gets enough attention, so I'll direct your attention to a single character who really is rather minor. But memorable. Each time he mentions the King, whom the band of rebels on the fringes of the kingdom are ultimately protecting against invaders without his knowledge, he references great deeds, offspring, and usually compliments his loins. He includes it in his speech as easily as breathing, as a substitute for "In the name of King George" sort of thing. (I apologize, I cannot remember the character's name--my copy is lent out at the moment.)
Which gets us into characters. If your main character is a good Catholic girl, she probably isn't going to swear. Maybe she'll say a quick prayer or involuntarily reach for her rosary or cross her heart. Your love interest is a badass biker boy who uses creative swears because he grew up with a senile grandfather who never had an end of them (most are about fifty years out of date as well). The best friend character often switches to chat speak so she can say more in a shorter period of time. The brother character speaks in short, slow, to the point statements because he is a man of few words (cliche of course, but all us girls have a soft spot for the caveman).
I'm mainly talking about swearing, aren't I? It's one of the most telling aspects of voice and dialogue. But it extends, as I briefly touched on with the best friend characters in the example above, to speech, mannerisms, how a character processes information before doing something with it.
My advice, as always, is to read and pick apart a successful novel, or one that you admire. Watch movies even (not all, of course, they're not all equally successful in this). Star Wars you have people who believe in the force using "May the force be with you" (and that great scene in A New Hope when Han tells it to Luke for the first time, imparting so much respect into that simple line) and Yoda of course with his awesome speech patterns, and so many little things like Leia telling Han she'd rather kiss a Wookie (as opposed to a dog or pig on Earth).
And pay attention to your own characters. What do they do in times of stress? What do they do during their Eureka moments? How do you, the narrator (whether in first or third person) convey that to us, the reader? Is there anything you can do to be more successful?
The danger is, if you continue to use your stalk phrases (as original as stock photos), your work is going to be passed over as a pale imitation of greater genius. In contemporary, your characters will come off unoriginal and bland. In SciFi and Fantasy, I won't believe your world building, and even question if your setting is in fact another world, rather than dystopian or a weak version of Earth's medieval era.
Remember, it's the little things that count.
The key word is "little things." Don't overuse them. Don't make them blatantly obvious. Weave these things into your story so well that the reader won't know what's happening until they get to the last page and scream "Merlin's pants!"