 Nimoy , in addition to his directing duties , appears towards the end of the film as Spock , but only appears in the opening credits as the director . Nimoy found the most difficult scene to direct was one in which Leonard McCoy talks to the unconscious Spock in sickbay , en route to Vulcan . Nimoy recalled that not only was he in the scene , but his eyes are closed , making it difficult to judge the quality of the shot or the actor 's performance : " It drove DeForest Kelley crazy . He swears that I was trying to direct him with the movement and flutter of my eyelids . " Nimoy was thankful the story required him to appear in a minimal number of scenes . The rapidly aging Spock , at the ages of 9 , 13 , 17 , and 25 , was portrayed successively by Carl Steven , <unk> <unk> , Stephen Manley and Joe W. Davis . Frank Welker provided Spock 's screams , and Steve Blalock doubled for Nimoy , so that a total of seven actors contributed to the role . 
