Spock's unwavering loyalty extends beyond his friendship to Kirk. The entire episode dealt with his loyalty to his first CO, Captain Chris Pike. The episode ends on a massive Crowning Moment of Heartwarming when the Talosians give their "gift" to Captain Pike.
And when Spock explains his need for deception instead of revealing everything from the beginning.
Kirk: Even though regulations are explicit, you could have come to me and explained.
Spock: Ask you to face the death penalty too? No, one of us was enough.
Late in the story, Scotty reports that the control room for the ship's phasers is being manned by one person. Lieutenant Stiles, the navigator, volunteers to assist as his first assignment was as a phaser controller. Kirk agrees and asks Uhura to take his place at navigation. What is not stated, or even commented upon, is that with Sulu already at his station, this means the Enterprise is now being operated by a black woman and an asian man. Nobody comments on this, nobody wonders about that, they just accept it as perfectly natural. With no dialogue or a heavy handed message, the show features just a natural acceptance across gender and racial lines.
The Devil In The Dark
Kirk and Spock resolve the situation by Shaming the Mob with the revelation that the Horta Monster Is a Mommy, and then propose a solution to resolve the situation, which works perfectly with both populations ending the story living and prospering together in peace. The head of the operation even calls Kirk to tell him how well they're getting along.
McCoy: My God Jim, I'm beginning to think I can cure a rainy day!
When part of the roof caves in and Spock hears, he tries to contact Kirk. When there's no reply, he instantly moves faster and even drops the formality of calling him "Captain", instead calling out "Jim". Spock may hide his emotions, but anyone can tell that for that moment, Spock was worried his friend had been killed.
The City On The Edge of Forever
Kirk and Spock finally find McCoy, who they'd been separated from for the duration of the episode. Jim and Bones shout each other's names and hug - and Spock runs forward at the same moment, shooting his arm out as if to join in the embrace. In the next shot, he's shaking the doctor's hand quite earnestly.
This troper found the emotional outburst that Spock nearly had after learning that he didn't actually kill the captain to be the biggest Crowning Moment of Heartwarming in the entire show. (Watch!)
It was the one and only time in the entire series where Spock smiles of his own free will and volition. Of course it was the best...
After Spock believes Kirk was killed, his farewell to T'Pau truly shows how much he cares for Kirk:
T'Pau: Live long and prosper, Spock.
Spock: I shall do neither. I have killed my captain and my friend.
A slightly fridgey one: Spock's Please Spare Him, My Liege! moment when T'Pring selects Kirk as her champion. He wasn't even supposed to be capable of coherent thought at the moment. It gets better when T'Pau demeans him for his human behavior and Spock doesn't stop. As much as he fears being humiliated in front of "his" people, he feared more to let Kirk die without attempting to save him.
Kirk's behavior adds extra heartwarming to the above. Spock gives up his chance of being accepted as a Vulcan, something he's wanted and worked at for years. What has Kirk been working at and wanting for years? The captaincy — the same thing he gives up. Not only does the Vulcan have someone he cares about more than his heart's desire, but also that person returns that friendship deeply enough to be worthy of it.
Spock asking Chapel to make him plomeek soup. Having vehemently (and very publicly) repudiated her for the same gesture earlier in the episode, knowing she has feelings for him that he can't share, he still sees that his illness is upsetting her and is able to accept her mercy and call her by her first name.
There's another, smaller one later on. Spock explains that the groom's 2 best friends are allowed to witness the wedding, and invites Jim and McCoy to come along. Really shows how much he considers McCoy a friend.
And Bones, ever the Southern gentleman, answers with a sincere, "I shall be honored, sir."
Early on in the wedding, T'Pau gives Kirk and Bones the option to back out of their participation in what is sure to be a very gruesome ceremony. They opt to stay and support Spock instead.
T'Pau: Spock has chosen his friends well.
A young ensign Garrovick freezes upon seeing a hostile alien that subsequently kills the rest of his landing party. Even though Garrovick already blames himself, Kirk comes down unusually hard on him, but only because Kirk himself faced the same creature eleven years before and also froze, getting many of his own crew mates killed. When Kirk discovers that the creature is immune to phaser fire, he seeks out Garrovick to deliver absolution:
Kirk: I'm asking for your military appraisal of the techniques used against the creature.
Garrovick: Ineffective, Captain. I realise, Captain, you did everything you could do. I know that. It's just that nothing works against a monster that can do the things that thing does.
Kirk: And Ensign, what is your appraisal of your conduct on the planet?
Garrovick: I delayed firing.
Kirk: And If you hadn't delayed firing?
Garrovick shakes his head and looks away.
Kirk: No difference, Ensign. No weapon known would have made any difference. Then... or eleven years ago. Report for duty, Ensign.
Garrovick: Yes, Captain. Thank you, sir!
Kirk also tells Garrovick to seek him out if he wants to hear some of Kirk's experiences with Garrovick's father (Kirk's former captain, killed in the previous encounter).
Sarek: At the time, it seemed the logical thing to do.
The Ultimate Computer
When Kirk thinks he's been rendered superfluous as a captain, Spock and Bones, each in his own way, rally round him. First Spock tells him firmly that "a starship...runs on loyalty to one man, and nothing can replace it or him;" then McCoy brings him a drink, and when Kirk bitterly toasts to "Captain Dunsel," the doctor puts a hand on his arm and quietly corrects, "To Captain James T. Kirk." What follows is an iconic Kirk speech that's a Crowning Moment of Heartwarming in its own right:
Kirk: Do you know the one: "All I ask is a tall ship and a star to steer her by...?" You could feel the wind at your back in those days, the sounds of the sea beneath you. And even if you take away the wind and the water, it's still the same. The ship is yours, you can feel her...and the stars are still there.
This is followed up years later in Star Trek V: The Final Frontier as they approach the Enterprise-A in the shuttle, where Spock and Bones, bickering couple that they are, argue over who the author of the poem was. It's called back even later in Star Trek: Deep Space Nine, as the dedication plaque of the USS Defiant actually bears the same John Masefield quote.
Spock and McCoy are having another discussion before Spock leaves. Spock suggests that since McCoy can't accept his way of dealing with the situation, he could "Wish me good luck." McCoy doesn't. Then, when the Vulcan is out the door and can no longer hear him, he says, "Good luck, Spock."
Spock and Kirk both praising the crew of the Enterprise in their personal logs.
This episode was just filled with heartwarming. First there's McCoy rasping "You've got a good bedside manner, Spock," as Spock tenderly cradles his face. Then when he regains consciousness to find Gem absorbing his injuries, he tries to push her away, physician to the last:
Bones: Don't let her touch me. She'll die. Jim... I can't destroy life, even if it's to save my own. I can't. You know that. I can't let you do it.
Also, earlier in the episode, after McCoy had drugged an injured Jim to sleep, Gem was watching Spock, who was looking worriedly at his sleeping captain. Not quite understanding the emotion, she reached out and touched Spock's shoulder, before finally realizing why. The smile on her face afterwards says it all.
Whom Gods Destroy
Kirk: They had a dream, a dream that became a reality and spread throughout the stars. A dream that made Mr. Spock and me brothers.
Garth: Mr. Spock! Do you consider Captain Kirk and yourself brothers?
Spock: Captain Kirk speaks somewhat figuratively, and with undue emotion. However, what he says is logical, and I do, in fact, agree with it.
Another example from the same episode. Kirk tries to talk down the maniacal Garth by reminding him of what a hero he is to the Federation and to Kirk, personally, for being the model for all other starship captains. For an instant, Garth looks and sounds normal. "I do remember that. It was a great responsibility, but one I was proud to bear." Unfortunately it does not last, but after Garth is subdued and cured of his insanity, he acts as if he is waking from a dream and the first thing he does is offer a handshake to Kirk.
Garth: "Should I know you, sir?"
That Which Survives
Spock's extreme Vulcanness is implied to be a sign that he's worried about Kirk. Then, later in the episode, he refuses to take Scotty's The Needs of the Many offer to jettison a malfunctioning part (and him with it).
Requiem For Methuselah
In the otherwise clunky "Requiem for Methuselah" after Kirk is emotionally devastated by the events of the episode, Spock and McCoy come to his quarters. McCoy, after delivering his final report, proceeds to upbraid Spock for his lack of "love". As McCoy departs, Spock leans over his captain's sleeping body and performs a mind meld with just one word: "Forget." Truly, this is the moment where we see just how deeply Spock cares for his captain... and his friend.
McCoy: Don't be a fool! My legs and hands are frostbitten; I can't feel my feet. Alone you have a chance. Go, try to find Jim.
Spock (pulling McCoy to his feet): We go together!
"The Devil in the Dark" when you know that William Shatner received news of his father's death the day before filming, and was scheduled to ship out later that day. Spock's personal space bubble is as small as it ever was in this episode, as Leonard Nimoy lends Shatner his support. Shatner later recounts his gratitude for the support of the other actors, particularly Leonard Nimoy and De Forest Kelley, likening it to the way elephant herds will converge around the bereaved, offering silent comfort.
Kirk and Uhura's kiss in "Plato's Stepchildren" — not the first integrated kiss broadcast on Television note That was I Love Lucy but still a big deal. The best part about this was two versions of the scene were supposed to be filmed; one with the kiss and one without. But both Shatner and Nichols purposely messed up the second take so it couldn't be used.
Some people might not know from watching the show. But James Doohan felt a great deal of animosity toward William Shatner for many years. However, while writing a book on the original Star Trek, Shatner asked Doohan if he'd like to collaborate with him on the project. Doohan agreed and both men appeared to walk away from the experience on amicable terms.
After the first season, Nichelle Nichols decided to leave the show and turned in her resignation to Gene Roddenberry. However, that weekend she met one of her biggest fans, who told her how important her role as Uhura was and convinced her to stay on. The fan's name? Martin Luther King Jr.
James Doohan received a letter from a fan that he recognized as a suicide note. He invited the fan to the next convention, and the one after that, and several more. After a time he lost contact with her, and had also lost her address. He had no idea what happened to her until she showed up at a convention again, thanking him for his support and telling him she'd just finished her Master's in electrical engineering.