Dialogue is conversation.
Or, to put it a better way, "Dialogue's conversation."
In other words, dialogue consists of the words people say, written down so that we, the readers, know what they say. And an English Lesson it ain't. What it is, is a faithful representation of how your characters actually talk, warts 'n all. With ~gasp~ adverbs, contractions, imperfect grammar, syntax, the lot.
One word of warning, though. If you want to lose your readers very fast, do not write in dialects! It's hard work reading dialects. Even harder, your work has a global reach, a global audience, for many of whom English is not their first language. Imagine how hard reading a dialect would be for those folk. Having imagined it, throw dialects away.
I was struggling to write an example, then I thought, use a search engine! Here is a Black Country translator! And it has no clues on pronounciation. Try this video for that.
That's Black Country folk translating their dialect into relatively clear English. So, setting a tale in the Black Country, how do you solve the conundrum? The answer is something like this:
Peter spoke in the local dialect with a very broad Black Country accent, one of those you had to tune in to before you could understand a word. "[example of dialect]," he said for the fourth time, ever more patient since I couldn't understand him. I got it, most of it, on the fourth try.
Then you revert to the normal English your characters speak, perhaps dropping an H, or using 'sumfink' instead of 'something'. A great example of that is Michael Arram's The Chav Prince, where the dialogue is pretty much at the limits of how far you can go in odd spellings. Michael hasn't needed to describe the accent, though. Justin's is penetrable. So it's horses for courses.
People do not speak the way we were taught to write by our esteemed English teacher.
"I am going to the shops, now," Gareth said. "What would you like me to get for you?" Just doesn't ring true as speech. Read it out loud. Now write what you just said. Contract 'I am' into 'I'm', or even drop them and start with 'Going'. Don't go overboard or you'll end up with "Going to the shops now, what'd you like?" which is a bit shorter than you might want.
Make it natural, flowing. Let folk interrupt others. Add in pauses by whatever mechanism you feel appropriate. Recognise that conversations can have periods of silence.
Do not, please, please do not, use names everywhere. Look at any conversation. How often do you use people's names? Use them sparingly. And, if you need to direct words to a different character, show us:
Geoff turned to Peter in surprise. "Why did you say that?"
We know who as speaking and to whom, but Geoff never used Peter's name. And we showed the reader that Geoff was now looking at him. Using Peter's name often within the dialogue stops the flow, makes the dialogue judder.
One rule that may not be broken. Every new speaker gets a new paragraph. This means you need to design the linking elements of your dialogue. You have the keyboard, so it's up to you to manage this in the correct way.
Who's speaking at any point in this example? It's the first paragraph of a story presented just like that on a well known story site. Already I clicked away from it because I find it unreadable.
When [Scarlett] got up in the morning she went down to breakfast. Her father and mother made their famous poached eggs with a basil dressing. Scarlett savored every bite. "What are your plans today? Scarlett's father asked. "Since its Saturday I need to finish my report for History class, and my essay for English class." "This means I need to go over to the school to pick up my books." "Which I forgot." Scarlett looked upset. "Honey it's alright, after you eat, take dad's car and run over to the school and get your books." Scarlett smiled. She finished her breakfast and went upstairs to get dress.
This leads me to punctuation, a veritable minefield. I had an English education, but it followed US rules for punctuation within speech marks, inverted commas, especially Rule 4. For completeness the British English rules are here.
We have one more issue to contend with. Our characters not only speak, they think. Thinking is an internal dialogue. I choose to differentiate thoughts by using the single inverted comma, from the speech which has the dual inverted comma. Others use italicisation. There is no obvious rule here save for consistency, and unambiguous writing.
Very few old folk sound like kids. Very few kids sound like old folk.
Sentence structures are different. Phraseology is different. The feel of the dialogue is different. Which means this is your job. Make the dialogue appropriate for the age and education level you have chosen for your various characters.
Well, that cuts two ways. What I mean is, how do you handle it if one character has a lot to say during their turn to speak, has more than one paragraph to say, wants to convey multiple ideas in one hit?
You'll need to be inventive. As long as we understand it's the same person speaking, try things to see how to do it. Use a Beta Reader to help you. If they know it's the same person, split away into paragraphs. Maybe introduce the long speech so we know it goes on a while?
Yes you can, and yes you may; you may mix direct and reported speech.
"It's nice to see you here again Jacob, but what's the emergency? You look as if you haven't slept in a week."
I won't go into how I told her, but I did. And she gave me a choice. [the paragraph continues way past our example]
Jacob could have rehashed the thing he had just told us all, and we would have been bored witless. So reported speech was used here as a shortcut to show that she had been told. The plot then moved on without doing the same thing twice.
There is, probably, a need to differentiate text messages from dialogue. The simplest methods are always the best methods. Changing named fonts fails. Apart from anything else it makes the story judder. Equally, the use of pure italics also fails, especially if you use italics elsewhere in the body of the story.
There is no established convention. THat doesn't stop us from establishing one. All we need is an indicator of the direction of flow of the conversation. We have characters for that: < and >. All we have to do is to make the reader aware.
My phone pinged. It was a text from Peter:
< We ok for Friday?
I had to check my diary
> Diary's clear
< Not sure that's what I asked, doofus!
That's simple, clear, and differentiates between normal face to face or phone conversation and text messages. Add italics if you must. I prefer not.
Remember, the last thing we want is the fact that it is a text message conversation to intrude into the reader's brain. We just want them to known it, easily and subtly
A reader usually laps up stuff told to him via dialogue, much more readily than he does from the story's narrative prose. It operates as a direct injection into his awareness of where he's at with the story. He picks up things directly from 'listening' to what the character's saying. It's therefore a good vehicle for giving the reader, story info and details, and reminding him, and reaffirming stuff he's perhaps been told some while ago. He will tend to accept and digest it more readily, because he's 'overhearing' it from the speech of a character, first hand. Here's an example of info dialogue:
"Wow, awesome. It's frikking huge, I can't even see the mast top. Where've you come from in it?"
"She's a one fifty foot sloop, not that huge with what's in the harbour. We're fifteen days out of San Juan, Puerto Rico."
And in third person narrative:
Rory goggled at what looked like a large ocean going yacht and wondered where it had come from. Nicky told him they'd come from Puerto Rico.
I've downplayed the narrative, but in comparing the two examples, see how you're not only given more information in the dialogue form, but it's told to you in a lively intimate manner. Rory's obviously delighted and very impressed and Nicky is cool and proud. In tandem, we might also imagine them eyeing each other up during this transaction, if the story bears out that context. To create this in a full narrative would take a deal longer and seem, wooden - just information for scene setting.