Tip for Finding a Character's Voice
If you're struggling to get your character's voices right or you're finding they're all sounding the same, do a little side project having your characters write letters back and forth to each other. Change the voice to speak however they would with different characters, in different situations, and about different topics that'll be addressed throughout your story (i.e., a boss vs a friend, at work vs at a party, telling a funny story vs telling a sad story).
It's a bit of free writing to build your characters and get a better feel for them, but it's also a great tool to reference to make sure you're writing dialogue that's true to your character.
Using the right variations in voice can add depth to your character and keep dialogue from getting too flat or inappropriate for the setting. Likewise, if you have a character that's commonly inappropriate, you can explore the ways they would disrupt a situation.
It's also a good way to explore their inner thoughts, develop plot points, and build your world.
Here's an example from my letters (these are not real things that happen in my book, so no big deal that I'm sharing them):
POV character to her twin brother (very close, casual relationship):
I'll ask Gritty to treat ya' with mercy, because somehow ya' spent every free minute pesterin' me at work and I never taught ya' to cut a wedged tenon. Next time you go drinkin', tell Arthur I'm not coming around because I passed. It was good for three pints when I tried it, but I'd guess ya' could get a couple more if you wail about it a little. He'll be feeling too seedy in the morning to remember I'm supposed to be underground, and all the guys up there have pulled the same shite, so they won't rat on ya'.
Same POV character to her aristocratic girlfriend:
I don't figure you would like that much. You think you're not keen on all those hands you have at home, but it's thorny up in the mountains. Apologies for saying so, but you're not exactly a master of common man's work. I'd expect we would need some goats, and they would sooner freeze than lift a foot to keep warm if you're asking. I can't picture you minding a garden or sewing a dress in hand, but hell, you've floored me before. All that's to say nothing of the cold up there. I don't figure a lady from Summer would fare well up in the mountains.