I wouldn't deliberately attempt to imitate any celebrities, but the act of portraying the NPCs would often remind somebody of one.
For instance, I once had a paternal Paladin {oooh, alliteration!} NPC that would appear to offer guidance and plot points. I played him as somewhat elderly, with a gentle disarming wit. A player pointed out that he sounded like Captain Picard, and that became his defining characteristic.
Another time a player decided to play his Elf Swashbuckler like Val Kilmer's Doc Holliday from "Tombstone"; all laid-back and slightly inebriated. He ended up affecting a soft Southern accent, and the group spontaneously adopted that as "the way our Elves sound". It was pretty cool, sometimes our session would end up sounding like a Tennessee Williams play!
Ah, good times . . .