No names, but not long ago I read a novel by a heavy-tech science-fiction sort of author that bugged me. The writer elected to tell the story all in first person, but with multiple narrators, about eight in all. Each one took turns in telling his/her parts of the story.
But - here's the rub - they all sounded exactly alike.
That is, nothing in the style of Mr. A's narrative in the least distinguished it from Ms B's, and so on. The only way of knowing who was telling the story at any given point was to go back and see whose name was in italics at the beginning of the current section.
And these were not clones.
Everyone has a distinctive voice. Think of your phone ringing, and you answer without even checking the caller ID. If it's a friend of yours, chances are you don't need the speaker to introduce himself or herself. Three words and you know who it is, just from the voice.
That should be applicable to first-person narrative, too. The speaker should be immediately identifiable by diction and syntax and attitude - by voice. This isn't rocket surgery, folks.
Look at some first-person narrators who do have a distinctive voice: Huckleberry Finn. Jim Hawkins and Dr. Livesey in Treasure Island. Bertie Wooster in any Jeeves novel (but one; Bertie was left out of Ring for Jeeves, and it is not the usual sparkling nonsense). Philip Marlowe. Kinsey Millhone. Jane Eyre.
To pull off a convincing voice, you must imagine yourself into the skin of your character. On the most superficial level, you can use tricks of style: "I reckon I'd better start in to tell y'all about..." or "I was nurtured to be a gentlewoman, and, alas, that is what I have become."
But to create a really convincing speaker, you have to put on those shoes and walk around in them and see the world as that character does. You have to be an actor as well as a writer.
It all starts by being aware. Remember that your narrators must have their own identities; they cannot be carbon copies of each other; they must not be dull or be made so by a lack of differentiation.
A writer friend of mine once complained that I had a fairly colorful minor character in and out of only one scene in a novel. "What's his name?" my friend asked.
"I didn't give him a name," I said. I mean, he was in for maybe five hundred words of a ninety-thousand-word novel.
"Dammit," my friend thundered at me, "Everybody gets a name!"
He was right. If you want to engage readers at all, each and every one of your characters deserves a name.
And a voice.