This is going to sound utterly absurd, but here's a joke my ex and I used to do to determine if dialogue was distinguishable between characters.
I call it the bob test. Change anything used to tell it's a different character to Bob. See if you can tell alone by dialogue if the characters are different.
Okay why such an absurd idea? Because people talk differently and have different speech patterns . This is a fact.
How many times have we said in our live times, "Yeah that sure sounds like something (insert name) would say."
People tend to get hung up or use certain words constantly like a broken record. You might have one character always swearing or using a light swear.
"Yeah well it means well but thing's gone to hell," Bob said to Bob lighting up a fag.
Bob grabbed his temple and groaned. "Look, I get that but just because she acts that way-"
"It's a load'a bollocks yeah hear? She's-"
"Playing me like a fiddle..."--Bob sighed pushing back his hair--"Yeah, yeah, Yes I get it. I know your opinion of the foul witch Bob.You don't have to tell me, again!"
"See I'm just worried about yeah mate. She got yeah by right by the bollocks. Just right there. You see right there mate. I think yeah screwed."
I hope my hastily and crappily written example proves a point. Two people never sound the same unless the culture mandates it.
Just walk around go to a store and buy a beef stick, a slushie, something and listen to people ramble on and on about things. You'll see how fragmented and the same sentences of people are.
A friend once told me, "You write how you talk, it's crazy." And I took that as such a compliment. (Well except I do have a stutter so if I repeat the same word fifteen times in row while making a post please let me know immediately.
)
Look for flow and organic. Look for the natural. You can put a lot of emphasis with a few words. Action beats help.
"Yeah." Bob frowned.
"Yeah?" Bob said reaching into his pocket.
"
Yeah!?" Bob snarled as he broke the beer bottle against table.
"Ye-yeah?" Bob gulped as she traced his collar with her hand.
I am not the best with grammar. Though I feel it's more a issue of using the proper punctuation than grammar. Many people disagree with me. FOR ALL MUST BE THE SAME AND WRITTEN IN FLAWLESS QUEENS ENGLISH WITH NO DETAILS AND EVERYTHING MUST BE KNOWN. Sorry had a flashback to another writere forum that did waaaay more harm than good.