One of my all-time favourite comedy films is called Getting Sam Home. It’s a feature length episode from Last of the Summer Wine. In one scene Sam is being driven home from hospital in a fish and chip van. Pondering on his imminent death, he takes a puff on a cigarette and says, “What about me ashes?”
“Flick ’em on the floor, why don’t you,” replies Norman Clegg.
A southerner like myself would be more likely to say, “Why don’t you flick ’em on the floor.” Exactly the same words, but in a different sequence. It’s an example of how the use of words - the idiom - can define the speaker: northerner or southerner. And it works as well on the written page as it does on the television screen.
Use of dialect or idiom does, of course, require the writer to have an appreciation of the way people speak in different parts of the UK. I find it helps if I can imagine in my mind a person from that region speaking my dialogue. Does it sound real? If not, why not?
You need to be careful not to overdo it. In The Gallows on Warlock Hill, twin sisters meet for the first time as adults. One has a southern counties English accent. The other grew up in Belfast. Their speech would, in reality, differ considerably. I chose to use one word as a way of defining the Irish girl’s speech. I had her say “yous” instead of “you.” That was little more than a hint to tell the reader about her way of speaking, but I think it worked.