Consistency in Dialog
One thing that will pull a reader out of your story quicker than most anything is dialog that doesn’t fit: a character who doesn’t have the education for the words they use, formality from a street urchin, contemporary slang in a historical romance, etc.
It’s easiest to miss this in secondary characters. As a result, they will begin to sound like one person playing different parts.
How I combat this:
During one of my many revision passes, I take each chapter by itself and set it aside with my trusty plethora of highlighters. Then, I go through the chapter, assigning everyone in the chapter their own color (as I go) highlighting all the dialog (as you can see up top). This helps me to see each piece of dialog assigned to the character more clearly. It helps to define their individual voices
Hope this helps if you find yourself having the same problem.
Back to the trenches!!!