Walter Koenig's Pavel Chekov
It's hard to guess whether Mr. Chekov would have remained a consistent character on the show if not for George Takei's absence from a good chunk of season 2. NBC allowed the actor time to film The Green Berets, which meant most of Hikaru Sulu's lines went to Chekov. This allowed Walter Koenig to build Chekov beyond what might not have been possible with Takei's absence.
Early on in season 2, Koenig played Chekov as inexperienced but intelligent and willing to learn. He was confident at Mr. Spock's station, and Chekov is the Vulcan's protegee much to Dr. McCoy's dismay in episodes such as "Who Mourns for Adonais?"
Throughout TOS and its accompanying six movies, Koenig gave us a character who was changeable or often contrary; bright but sometimes too eager for his young age. This eagerness caused Chekov to get his butt whipped several times in seasons 2 and 3. You've heard of the horror genre's Final Girl? For fans who read Chekov fanzines and watched TOS, Koenig was our scream king! (Continued...)