Me (Voicing an NPC) – “What is it that you want from me?”
Player – “Woah, is that (BBEG)?”
Me – “What? No, he’s just a guy. Just a friendly guy.”
Player – “He sounds a bit like (BBEG).”
Me – “… dammit.”
Voicing NPC’s have never been something I’ve been particularly strong at. It takes practice, a level of comfort with your players, and some tricks and tools to draw from. I myself try to practice voices when I’m commuting and stuck in traffic. Sure I may look like a crazy person, but we’re in the 21st century. Maybe people look at me talking to myself and assume I’m on the phone (hands-free of course) rather than voicing a psychotic henchman for my campaign’s BBEG.
DM Dave put up a great video discussing this very topic. Check it out, won’t you? Hopefully, it helps you as much as it will (probably) help me.