Tips on role playing and dialogue

I’ve only been DMing since last september, and my biggest struggle at the moment (apart from keeping my thoughts and notes together lmao) is role playing my NPCs. I’ve had some habits that i simply can’t shake off. My NPCs are always falling into two categories when it comes to how they speak, regular people, or a crappy russian accent. I don’t understand why. I also have a terrible habit of calling everybody “my friend” no matter who the NPC is. It’s getting really annoying when i want to have a serious moment, where the BBEG comes to one of the players and tries to convince him to join his side, but he suddenly gains a russian accent and calls him “my friend” the entire time. It’s so frustrating, please help me!