Voice is everything in fiction and every voice of each character a writer creates is another voice, or role--a part to be played, and every voice must be as distinct as the character's fingerprints.
To that end your characters must not sound so much alike, as if they had all gone to the same finishing school (gramatically spot on, Reginald) or likewise all sound as if they have had the same street jibe crammd into their heads so that only THEY know what is being said to one another. Being a book doctor, I have seen it both ways.
A play wherein only one character speaks my language while all the others speak in such "jive talk" as to make no sense to the general reader. A novel in which each character speaks in exacting gramatical certitude.It does not work.
Every character should have some quirks of a verbal nature. You don't want all your main characters walking alike so why are they all talking alike? They don't all come from the same Stanford or Harvard background, do they?
Some don't give a blankety-blank what comes out of their mouths, while others are guarded, and a man of few words is quite the opposite of a verbose fellow. Sometimes an overweight person talks overly, sometimes just the opposite. Character is built into language patterns. Work hard to both see and hear the language of each character. Let him or her tell you how they talk.
Don't force your patterns onto your creations. A guy working the back of a trash truck is gonna talk some funny way different from the professor or the cemetery administrator, but the grave digger and the trash-man might very well speak the same "language" to one another as the prof might to the administrator.
Key into the sound of the bell being rung by one character as opposed to the sound of the bell rung by the other guy. Take in their psychology, their history, their upbrininging and station in life.
It all comes through in the voice...in the dialogue. "I believe it is coming on a storm," Hector, the professor said to Elaine.
"Comin' up a storm, eh what?" asked Liam, the caretaker to Elaine.
"I quite agree," replied Elaine in both instances, but she might give Hector a look, whereas she keeps her eyes to herself with Liam.
For more on dialogue dig through the archives of ACME and you will be surprised to see that the topic has come up before, and there are many examples in the archives here.
Happy Writing one and all,