The original script called for Spock to karate chop Kirk to subdue him. Leonard Nimoy felt that this would be an uncharacteristically violent act for a peace-loving species like the Vulcans so he came up with a pincer-like grasp on the neck that has since become known as the Vulcan Nerve Pinch and become one of the character's most famous gimmicks.
According to Grace Lee Whitney, while shooting the scene when a distraught, tearful Janice Rand accuses Captain Kirk of trying to rape her, William Shatner slapped her across the face to get her to register the proper emotion. As they shot the attempted rape scene days earlier, Whitney couldn't get into the same emotion successfully, and it was Shatner's "solution" to the problem.
This is one of the few times in Star Trek where it can be seen that the middle finger on actor James Doohan's (Scotty's) right hand is missing. Doohan lost the finger when it was struck by a bullet or shrapnel during the D-Day invasion in 1944. He took great pains to conceal its absence during the series, but his full right hand can be glimpsed briefly when he reaches into the box holding the snarling alien dog.
The evil Kirk was given eyeliner to enhance the starkness of his gaze.
Richard Matheson said, "I thought William Shatner was brilliant. I loved what he did. He carried the whole thing. I was a little sorry that Gene Roddenberry put so much emphasis on the crew being stuck on the planet. (...) But I liked it and I was very satisfied with the production value."