One of the hardest things to do when writing fiction is to create unique voices for each character. If you make all the characters sound alike it makes the writing flat and uninteresting. It also makes it difficult to get to know a character -- they all sound like the same person. When I teach my fiction writing workshops, I always teach writers the following concepts to help create a voice for each character.
Phone a friend -- I'm being cute ... what I really mean is pick someone you know in real life. Hear how he or she talks. Take this person's voice and apply it to one of your characters. If you don't know someone who would "fit the part" then take an actor or actress or even a public figure and listen to how this person talks. Mimic their inflections and phrases in your character(s). My biggest insight: really listen and repeat. People have their own ways of saying things. So, you want to capture the unique essence of the voice.
Formalism in modern writing sounds ridiculous or who talks like that? -- turning modern characters into "narrators" doesn't work. This dilutes the voice into a bland vanilla-sounding, uniform voice. Rarely do people talk in formal English. Most people are colloquial and use vernacular of some kind. Even the most educated character will use some kind of idiom to express something. So don't make your characters talk like they are nothing more than narrators.
Pick a unique phrase or behavior -- give your characters their own vernacular. Do you know someone who says a particular phrase all the time like "it's all good" or "and shit" or "whatever" ... these can be your character's "brand" phrase. Do you remember on the series Entourage, Ari Gold used to say "BOOM" -- and thus, a whole new word became the way to express something. The same goes for your characters. Give them a choice word or phrase that defines that character.