Younger version: Zachary Quinto (2009's Star Trek)
Older version: Leonard Nimoy (the original 1966-69 Star Trek series, the first six Star Trek movies, and the 2009 Star Trek, among others)
Why it works: It's hard to imagine anyone could add anything to Nimoy's iconic portrayal of the logic-driven, half-human Spock. Yet Quinto's early Spock plausibly reveals how the young Vulcan might still have had difficulty suppressing his emotions. Plus, he looks the part; seeing them on screen together in the 2009 film makes the transition even more seamless.