We all speak differently. Though most adults have a large vocabulary, they rarely use all of the words they know in regular conversation. Your characters should be the same -- while we are all tempted to create a larger than life character who owns the room and the scene they are in -- it is simply not realistic.
Personally, I'm a fast talker. I also talk with my hands which can be interesting for people around me when I have a Bluetooth headset stuck in my ear. I'm sure half my neighbors think I'm insane.
Think about the people in your life and put them in situations -- you can picture them moving around, talking, and acting because you know them. This is how you should know your characters. They are individuals in your mind and in your narrative. They have a favorite curse word, they answer the phone a certain way, and they call their lover "babe" instead of "sweetheart".
Word choice is important. How people phrase their thoughts when they are angry, excited, or sad is individual.
Personally, when I get emotionally hurt - I can barely speak. I stumble over my words, hyperventilate, and often stutter. My husband's voice drops -- gets deep like Barry White when he's angry and soft, barely audible when he's just sad or upset.
My grandmother's favorite exclamation went like this: "Now, ain't that a shame." She also favored: "Bless his heart." Both phrases within the context of coming out of a southern woman's mind can mean a multitude of things. "Bless his heart" could mean anything from "I feel sorry for him" to "he was so stupid, I'm not surprised he died doing something foolish."
You can give your characters depth and create real intimacy between them AND with your reader if you pay attention. Don't allow them to sound all the same, use different phrases, different speech patterns and rhythms for each character you create.
Use caution, however, because you want to use such tricks and tools sparingly to avoid turning your characters into stereotypes.