People who went to the same school, or were brought up together, or work or play together, tend to mimic each other, not just in their mannerisms but also in their vocabulary and how they speak, without even noticing.
In fiction, though, a reader needs to be able to distinguish between characters, and this can be achieved by making sure the various characters speak differently.
So, rather than just varying them in appearance – height, weight, gender, eye colour, hair colour, social standing, etc – it’s important to establish their manner of speaking:
When writing, hopefully this differentiated style will be automatic. If not, this aspect of your writing can be addressed to good effect during the editing of the first draft.