It was originally planned for Spock to kiss the Romulan Commander, but both Nimoy and Linville agreed they needed to use something different than normal "human" love expressions, and suggested the hand contact that was eventually used instead. Nimoy, in fact, wrote a long letter of complaint to Gene Roddenberry about this issue.
The Romulan/Klingon bridge set is reused from "Elaan of Troyius".
The Romulan insignia designed by Matt Jeffries composed of a yellow hexagon in the center with three colored spokes coming out of it, is seen in the corridor of the Romulan ship. This symbol was intentionally made to look similar to the Klingon insignia because in the shots of the ships you could somewhat make out the Klingon markings.
The original script didn't have the Romulans in Klingon ships. Due to fact that they had a lot of money invested in the Klingon model they needed to get their money's worth out of it by using it again in this episode.
This is the third and final appearance of the Romulans on the series.
The Klingon D7 Type vessels used by the Romulans in this episode didn't got a Romulan type-designation in this episode. Later in the books those vessels were called Stormbirds. The designation Warbird was never mentioned in Star Trek until The Next Generations Episode "The Neutral Zone".
The Romulan cloaking device is a reuse of the Nomad head from "The Changeling" and a power globe from "Return to Tomorrow."
Chekov: I've completed the assignment, Captain. "A theoretical incursion into..."
Kirk: Yes, Mr. Chekov. I can read. As usual, your theoretical evaluations don't tally with mine. Return to your duty. I'll let you know when your work is satisfactory.
Romulan Commander: Captain Kirk.
Kirk: Commander, I'm honored.
Romulan Commander: I don't think so, but we have an important matter to discuss. And your superficial courtesies are an overture to that discussion.
Romulan Commander: I have heard of Vulcan integrity and personal honor. There's a well-known saying--or is it a myth--that Vulcans are incapable of lying?
Spock: It is no myth.
Romulan Commander: Then tell me truthfully now, by your honor as a Vulcan, what was your mission?
Spock: I reserve the privilege of speaking only when it will not violate my honor as a Vulcan.
Romulan Commander: It is unworthy of a Vulcan to resort to subterfuge.
Spock: You are being clever, Commander. That is unworthy of a Romulan.
Romulan Commander: There is no force I could use on a Vulcan that would make him speak. There are, however, methods that are completely effective against humans, and human weaknesses.
Spock: They would prove ineffective against the Captain.
Romulan Commander: Then they will leave him dead, or what might be worse than dead.
Scotty: This is Lieutenant Commander Scott. The Enterprise takes no orders, except those of Captain Kirk. And we will stay right here until he returns. And if you make any attempt to board or commandeer the Enterprise, it will be blown to bits along with as many of you as we can take with us.
Romulan Commander: You humans make a brave noise. There are ways to convince you of your errors.
Romulan Commander: You are a superior being. Why do you not command?
Spock: I do not desire a ship of my own.
Romulan Commander: Or is it that no one has offered you, a Vulcan, that opportunity?
Such opportunities are extremely rare.
Romulan Commander: For someone with your capabilities and accomplishments, opportunities are made... and will be. I will see to that, if you'll stop looking on the Federation as the whole universe. It is not, you know.
Spock: That thought has occasionally crossed my mind.
Uhura: Doctor, beam aboard the Romulan flagship immediately. There's been an injury.
McCoy: I don't make house calls.
Romulan Commander: I neglected to mention I'll expect you for dinner. We have much to discuss.
Romulan Commander: Allow me to -- to rephrase. Will you join me for dinner?
Spock: I am honored, Commander. Are the guards also invited?
Romulan Commander: I hope that one day there will be no need for you to observe any restrictions.
Spock: It would be illogical to assume that all conditions remain stable.
Kirk: My neck feels like it's been twisted off.
McCoy: That's the Vulcan death grip for you.
Chapel: There's no such thing as a Vulcan death grip.
Kirk: Ah, but the Romulans don't know that.
Scotty: All right, Doctor, what's so urgent that you... Captain? Captain Kirk?
Scotty: Well, you look like the Devil himself, but as long as you're alive; what's it all about?
Kirk: Are those Romulan hostages still aboard the ship?
Scotty: They're in the brig, sir.
Kirk: I'll need their Romulan uniforms.
Scotty: (chuckle) Aye--it'll be a pleasure!
Romulan Commander: Romulan women are not like Vulcan females. We are not dedicated to pure logic and the sterility of non-emotion. Our people are warriors. Often savage. But we are also many other pleasant things.
Spock: I was not aware of that aspect of Romulan society.
Romulan Commander: As a Vulcan, you will study it. As a human, you would find ways to appreciate it.
Spock: Please believe me. I do... appreciate it.
Romulan Commander: I am so glad.
Romulan Commander: Mr. Spock. You do know I have a first name.
Spock: I was beginning to wonder.
Romulan Commander: Well, I do. Would you like to hear it? (she whispers)
Spock: How rare and how beautiful. But so incongruous when spoken by a soldier.
Romulan Commander: If you will give me a moment, the soldier will transform herself... into a woman.
Romulan Commander: Is my attire now more appropriate, Mr. Spock?
Spock: Commander, your attire is not only more appropriate, it should actually stimulate our conversation.
Romulan Commander: Oh, you must be mad.
Spock: I assure you, I am quite sane.
Romulan Commander: Why would you do this to me? What are you that you could do this?
Spock: First officer of the Enterprise. (she slaps him) What is your present form of execution?
Kirk: Commander, you'll forgive me if I put up a fight?
Romulan Commander: Of course--it's expected.
Spock: It is regrettable that you were made an unwilling passenger. It was not intentional. All the Federation wanted was the cloaking device.
Romulan Commander: The Federation? And what did you want?
Spock: It was my only interest when I boarded your vessel.
Romulan Commander: And that's exactly all you came away with.
Spock: You underestimate yourself, Commander.
Romulan Commander: You realize that very soon we will learn to penetrate the cloaking device you stole.
Spock: Obviously. Military secrets are the most fleeting of all. I hope that you and I exchanged something more permanent.
Romulan Commander: It was your choice.
Spock: It was the only choice possible. You would not respect any other.
Romulan Commander: It will be our secret.
McCoy: You're due in surgery. I'm going to bob your ears.
Spock: Captain, please go. Somehow, they do not look aesthetically agreeable on humans.
McCoy: Well, are you coming down or do you want to go through life looking like your first officer?
Kirk: I'm on my way!