1952: Zombies of the Stratosphere was Nimoy's first sci-fi role. Presaging Spock, Narab was an alien -- a martian. The Narab character was a very minor role, akin to Thug #2 or Henchman #3. But, Nimoy did get a couple of speaking lines and a few precious seconds close up on the screen near the end of the film. The sinister martian leader's rocket is shot down by the hero rocket man. The wounded Narab tells Larry where the nuclear bomb is hidden and how to disarm it, thereby saving Earth. Yay!
1954: Them! saw Nimoy in such a small role, he was not credited. He played the role of a sergeant in an army intelligence office. Still, he did get a couple of lines there too.
1958: The Brain Eaters saw Nimoy in a bit more substantial role. Again with a bit of presaging, the character of Professor Cole becomes a sort of hybrid -- half human, half alien -- when he is infected with the aliens. He gets a few speaking lines in which he speaks for the aliens. Regardless of the aliens' professed intentions of bringing a semi-benovolent tyranny to improve mankind's lot. The heroes are understandably unimpressed and proceed to electrocute the aliens.
Nimoy stayed busy in the late 50s, early 60s, with bit parts in westerns, such as Bonanza and Rawhide, or small parts in crime dramas.
That is, until 1966, when he played the character of Mr. Spock in the pilot for Rodenberry's proposed TV series, Star Trek. That pilot "The Cage" failed to turn into the series. A second pilot was shot, this time with William Shatner as the brash captain of the Starship Enterprise. Nimoy was the only member of the first cast (for The Cage) that carried over to the second pilot, still as Mr. Spock.
Over the next 40 years, Nimoy would continue to play Spock in one variation or another. It is amusing to remember that such a monumental legend as Spock started out as the humble Narab in a minor role in Republic serial in 1952. Rest in Peace, Mr. Nimoy. You've served sci-fi very well.