We don't all talk the same, so of course we don't expect all characters to do that in the books we read either. But neither is writing it as easy as one might think.
Where books are concerned, with regards to dialect, less is often more. A few key words or usage can give just the right amount of flavor to the dialogue. But if overused, one can make the recipe too overwhelming, the flavor so overpowering the reader will no longer want to partake of it.
We don't want the reader to have to work too hard to understand what a character is saying. Being too true to life can work against us.
So how do we do it? Well, we think of the dialect or accent we're wanting to convey and pick key words to use or change so the dialogue conforms to what we need thus changing the flavor to the slant we want, but not too much.
Let's play with the following sentence - "You shouldn't do that!"
Now to spice it up. Read the minor changes or word additions and see if it doesn't bring to mind different types of people.
"Ya'll shouldn't do that!"
"You bloody well shouldn't do that!"
"Ay Dios mio. You shouldn't do that!"
"Dude, you shouldn't do that."
"You, how do you say, should not do that!"
"Yah shouldn't do that, mate!"
"Bambino, you shouldn't do that!"
Though it was basically the same sentence each time, did you hear the different dialects in your head as you read them? Did your brain leap to certain conclusions about the characters who were saying them?
While any and all of the above could be said with a lot more of the words and styles of the different cultures, we don't need to. The author gave information with just a word or two which a reader could easily pick up and it didn't have to be overdone to get across.
As authors we're always trying to suggest flavors, paint pictures, but seamlessly, to draw the readers in deeper. And only sprinkling, rather than laying the dialect to thick will help do just that!
Have you ever read anything you felt was over spiced with the dialect?