A Non-Player Character (NPC), are all of the people and creatures the players encounter in the world presented by the GM. The GM must portray all of the NPCs.

Different GMs do this differently. Some merely portray them by the way the NPC describes things and using a standard vocabulary for a given type of NPC, like town guards.

Ohter GMs go all out and get into doing voices and facial expressions and the whole nine yards, much like a parent does when reading a bedtime story.

There is no right way for the GM to portray an NPC. As long as the players know which NPC they are talking to and everyone is having fun, you are doing it right.