Re: Assignment: Earth - Is it Really a Star Trek Episode?
Voiceovers are recorded in post-production, or at least separately in a recording studio, not live on set. So no, that couldn't be it.
Perhaps he was feeding lines from off-camera. Some actors prefer getting their cues from an actor rather than a script supervisor as non-actor's delivery can sometimes be pretty flat with little to play off for the featured actor.
Moe: I'll take the blonde!
Larry: I'll take the brunette!
Curly: I'll take the Black and Tan!
--Wee Wee Monsieur (1938)