delineating dialogue

Not very helpful.  There's something about the "voice" of a character that distinguishes him.  Throwing in slang, regional idioms, weirdly spelled version of a word, no one knows how to pronounce as it is spelled, does not help those readers who cannot guess what it is supposed to be.  For example, you need a character who has slight Scottish accent and how does an author spell how that character would say the word "do"? - a telltale sign to me that he is Scottish in that single word.  By "voice" I mean a more complete package of ways of saying things to indicate education, mood, attitudes, and even mindset behind that particular conversation.  And what of the ellipsis? I am of two minds about that. Halting and hesitant speech can mean different things, but that representation is only good when actual unsaid words are left out. What...can... I mean you, er, but...   I feel that written language fails to capture this, but it says something about a person's voice, at least in a particular circumstance .  Geoff Le Pard rule Five is not helpful and contradicts his initial advice about differentiating dialogue between characters:

If you have to recount back story in dialogue (and try not to) then break it up. People mislead themselves, they go off on tangents, they lose their thread. Use this to distinguish them, certainly but do it sparingly. Remember the benefit of dialogue in a narrative is to move the scene forward at pace, to help give tension to a piece, to introduce emotion.

The benefit of narrative within dialogue is to move the scene forward. Dialogue can be a distraction to the story unless the voices of the characters are telling something beyond  the story, and by "voice" I mean everything of which Le Pard mentions nothing.