10 Behind The Scenes Reasons For Star Trek Characters' Quirks
4. Scotty's Right Hand
In the Original Series and most of the TOS era movies, Scotty rarely allows his right hand to be in frame. His left hand features prominently in almost all shots, as James Doohan did not want to display his other hand. He had served during the Allied Invasion of Normandy on D-Day and, during combat, his right middle finger had been severed.
Although there are scenes in the early years where, for one reason or another, the hand manages to slip into frame, it would not be until Star Trek V: The Final Frontier that Doohan seemed to relax his attempts to conceal it. While he stands with Uhura on the bridge, discussing shore leave and about to tuck into a lunch of rations, his right hand is clearly visible, missing finger and all.
Though up to this point he had mostly used hand doubles during filming, there was a scene in The Trouble with Tribbles where the missing digit was noticeable while Scotty held a huge armful of Tribbles. In The Next Generation episode Relics, he also doesn't try to hide it while sharing a drink with Picard on the holodeck.