When you write dialogue, when you write dramatic monologues, when you write in first person, your character must have his own voice.
Voice is in part the distinct vocabulary, grammatical tendencies and spoken patterns which are unique to your character. A linguist would call this his idiolect.
Voice’s other part is made up of the interests and obsessions of your character: what sort of things they notice about the world, how they interpret them in relation to their previous experiences, and what emotional colouring they give events. We might call this perspective.
Firstly, a story: I am English, middle class, and I studied at Oxford. When I worked in a supermarket cafe in inner-city Cardiff (Wales) I’d get customers who would say, after half a minute’s banter about their breakfast at the till, “hang on, you’re too clever to be working here!” They didn’t say this because I had told them my life story,