The definition of diction is:
The accent, inflection, intonation, and speech-sound quality manifested by an individual speaker, usually judged in terms of prevailing standards of acceptability; enunciation.
Tone: any sound considered with reference to its quality, pitch, strength, source, etc.
Now read these two sentences:
“It would please me so to have you join me for a wondrous gala event at my place.”
“Could you join me for a birthday party at my house?”
There you have it, formal diction and a normal tone of inflection. If you’re writing a formal book on manners, you might use the etiquette of kings and queens, but if you’re writing to the reading public, you’re going to need to speak to them.
We have formal diction, used in a scholarly environment. This is where people of knowledge sit around talking in big words that have no meaning for the uneducated. This form is used in boastful conversations where doctors and lawyers want to show each other how smart they are.
Informal, used often in a normal environment. This is where people in a cafe speak using the language that they were raised to understand.
Colloquial, which is a word, phrase, or form of pronunciation that is acceptable in casual conversation. This is where you speak as if you are more educated than you really are. Or this is how you normally speak.
And then there is the colorful use of slang where you use words like “S’up” for “What's up?” or “Cool, man." Slang is usually street talk and fresh words are added with each new generation.
When you read a book, whether out loud or to yourself, you’re going to hear a tone in the writers voice. Whether they are using slang, dialect, or aggression, the tone is going to come through for the reader. You’ll hear it in a sympathetic tone: “I’m so sorry your dog passed away.” Or in an aggressive tone: “Michael, did you hit your brother, again?”
In the one sentence the sympathy is clear, in the other the mothers voice almost drips off the page. I said almost. You did get the idea right? The tone is the words that you are going to use to bring your character alive. To me personally, when you use a formal tone, you’ve lost me. Why? Because I was raised in the city and I know city-speak not a formal tongue.
We need to sound real to our reader and depending on your character that you’ve developed, only you can decide which tone or form of diction that they will use. Whatever you use, make it real to the reader. Allow them to be a part of your character instead of standing in the background wondering what the character is saying.
If you can nail this element, your dialogue will flow freely instead of sounding stiff.