I think it's about words people use or things they always add. Things like "Like" also a lot of people start every sentence with "So". Maybe other things like "Mate" "Dude". Two of my mates speak pretty similiar accent wise but one uses "Dude" and the other uses "Mate." A lot of people use "Babe" or "Hun," well they do round here. Some people swear a lot, some swear when only necessary and some don't swear at all. or will use words like "Fudge" or "Flip".
The thing is this is screenwriting so you don't need to write with an accent, you can simply introduce a character with an accent;
ANDY, mid 40's, well groomed, handsome with a COCKNEY accent... job done. (sounds like someone I know) lol
I have only just started screenwriting so the way people spoke meant a lot to me but that was because I've written a novel and obviously I can't just say someone had a strong accent I had to write it. An example of this is Trainspotting, the novel is written with really strong Scottish accents, which to me made it hard to read. A script however can just say the character has a strong Scottish accent but write it so it is easy to read and and understand.