IIRC, there are only two TOS episodes which feature the Romulans: Balance of Terror & The Enterprise Incident. I love both of these episodes and all of the Romulan characters in them. However, I must say that I have always found it odd that the Romulans in BOT spoke in a very aristocratic and somewhat Shakespearian manner:
COMMANDER: Danger and I are old companions.
CENTURION: We've seen a hundred campaigns together, and still I do not understand you.
COMMANDER: I think you do. No need to tell you what happens when we reach home with proof of the Earthmen's weakness. And we will have proof. The Earth commander will follow. He must. When he attacks, we will destroy him. Our gift to the homeland, another war.
CENTURION: If we are the strong, is this not the signal for war?
COMMANDER: Must it always be so? How many comrades have we lost in this way?
CENTURION: Our portion, Commander, is obedience.
COMMANDER: Obedience. Duty. Death and more death. Soon even enough for the Praetor's taste. Centurion, I find myself wishing for destruction before we can return. Worry not. Like you, I am too well-trained in my duty to permit it. Continue evasive manoeuvres. Now, back to the first course.
While in TEI, the Romulan Commander adopts more of a stereotypical almost cartoonish "we have ways of making you talk" language style:
COMMANDER: We have not even begun! There's no force that I can use on a Vulcan that will make him speak. That is a fact. But there are Romulan methods completely effective against humans and human weaknesses.
SPOCK: You would not resort to them, Commander. They would prove ineffective against the captain.
COMMANDER: Then they will leave him dead, or what might be worse than dead. But I will know your unspoken truths.