Well, since I'm facing this issue myself, I thought I'd do up a post about it so that we can muse together and I can share my thoughts.
What is manner of speech?
It is their tone of voice, the volume, how quickly they speak, their inflection, emphasis and pitch. Where film can actually let you hear all of these play out, books need to show you through clever use of punctuation and phrasing. Look at the difference between this:
“When I sit here and look at you there is nothing I would like better than to punch you in the throat.”
“When I sit here and look at you ... there is nothing I would like better than to punch you in the throat.”
“When I sit here and look at you, there is nothing I would like better … than to punch you in the throat.”
“Wh-when I s-sit here and look at you, there is n-nothing I would like b-better than to p-punch you in the throat!”
Read them out. How do they sound to you? Do you naturally change your pitch, the speed of the sentence? Emphasis and tone? And that's without going anywhere near the italics and how an italicised word changes the sentence. If you want to see how italics can play havoc with meaning by stressing certain words, just take one of those lines and repeat it over and over but with the stress on a different word each time. Different people would stress different words.
Then you've got sentence structure. This also requires punctuation changes but it's more than that. It's how wordy they are. How many frills, bells, and whistles they use. How many similes, metaphors, and how simple or convoluted they speak. Some might speak in overly long paragraphs that indicate breathlessness (though don't overuse these or you might strain the reader's mind) while others might be terribly succinct.
“I sit here. I look at you. I want to punch you in the throat.”
“I sit here, right, and I'm looking at you, and all I'm thinking about is punching you in the throat.”
“I can't stand sitting here and looking at you. All it does is make me want to punch you in the throat.”
Last but not least, you have the actual spelling changes to indicate accents. This is the last one I'm mentioning because, unfortunately, it's quite easy to just mess with the spelling and keep everything the same. While this tactic will still make the lines of dialogue recognizably different, there is, of course, further you can go.
“I can't stand sittin' 'ere and lookin' at ya. All it does is make me wanna punch ya in the throat.”
“I cannot stand sitting here and looking at you. All it does is make me want to punch you in the throat."
Finally, you've got the actual metaphors, slang, and similes they might use. In South Australia, people disparagingly talk about those with low IQ as being 'Mindas' as there is an organization called Minda that assists people with very low IQ. Even people who don't know about the institution will probably know the word. People in the Northern Territory, probably wouldn't use that term.
In Curse of the Rose / The Butterfly Lady (really must settle on a title), a lot of the slang used are either nautical terms, mystical terms, or terms related to the ghastly salt plains that raise the dead. “Salt it!” becomes a terrible swear word and polite people therefore avoid asking for anyone to pass the salt and instead ask for seasoning. It's not a taboo word like our F word or even the C word but it's still stronger than 'damn it' as it conjures up images of rotting flesh. Other phrases include 'barrel fever', 'loose canon' and 'having a cold soul'.
So, do you have any advice about how to make characters sound different and diversifying your dialogue?