Though other actors have now portrayed the character, we can’t think of Mr. Spock without first envisioning Leonard Nimoy. By now, most of us know that Spock was supposed to have a reddish tint to his skin, but there was so much more Gene Roddenberry had in mind for the first Vulcan, and one of those things would have excluded Nimoy from taking the role.
Along with the skin color, the alien with the pointed ears originally wasn’t going to eat or drink anything. In fact, he was going to have a metal plate in his stomach that allowed him to absorb energy. That meant that some type of energy would have had to have struck that plate. So perhaps phaser fire? Or something from the warp engines? It isn’t clear exactly how that would have worked, and fortunately, the writers talked Roddenberry out of this idea.
The biggest difference that would have prevented Nimoy from portraying Spock is that Roddenberry envisioned a little person in the role. (Source: The Fifty-Year Mission-The First Twenty-Five Years by Edward Gross and Mark A. Altman) He though that most actors were of average height, and they couldn’t do much to make him stand out besides the ears and eyebrows, etc. He wanted the green-blooded Vulcan to really stand out. Plus, having Spock be a little person would have also echoed the feelings Roddenberry had that size should not be that important.
And while it certainly would have added to the diversity of the cast, it’s difficult to think of anyone starting Spock’s journey but Leonard Nimoy. That’s not to say someone else couldn’t have taken on the role and done a good job with it, but because we saw Nimoy first, he will always be the first and foremost Spock.