Dramatic and comic dialogue is not conversation 鈥� it is there for a reason, it is honed and shaped and, from the writer鈥檚 point of view, purposeful.
But dialogue is not logical 鈥� characters, like people, do not necessarily or naturally express themselves in perfectly coherent grammar. Unless, of course, that happens to be something very specific to their personality 鈥� so not just the words they say, but who they are.
Great characters have an identifiable voice 鈥� they have tone, inflection, their own grammar, their own tics and tropes and ways of expressing themselves. Their voice needs to be individuated 鈥� to be particular to them.
Strong character voices are authentic. They express themselves, they aren鈥檛 mouthpieces for anything else (or the writer) 鈥� unless of course being a mouthpiece for something else is an intrinsic part of their character and the story鈥� So beware of characters suddenly making speeches or grand statements that don鈥檛 ring true.
If your character has an accent or uses dialect/slang, then write it in 鈥� but be sparing and be specific. When we first see them, indicate their accent 鈥� but don鈥檛 exhaustively write in a kind of phonetic version of it all the way through, because it can be impossible to read. Slip in specific words and terms only that dialect would use.
The biggest problem with most dialogue is being 鈥榦n the nose鈥� and being expositional. If the only reason for dialogue being there is to relate information to the audience, then think again. Find dramatic ways of making information significant in the moment and in the story. If you want the audience to realise a secret about a character, make the revelation of it difficult, with real consequences in the story.
Dialogue isn鈥檛 just about the words on the page - it鈥檚 about the things that are not said. The space between the words. The silences that speak volumes. The subtext of what鈥檚 going on below and behind the words.
