Mark Lenard plays the Romulan Commander, an apparent enemy of the Enterprise and its crew, however later in his career he played the famed role of Spock's father Sarek, and also played a Klingon in Star Trek: The Motion Picture (1979), making him the first actor to portray the three major alien races (Vulcan, Romulan, Klingon) in the Star Trek franchise.
Budgetary and time constraints prevented the make-up and costuming departments from dressing up each Romulan in Vulcan ears as it was such a lengthy process applying them. So they hit on the idea of giving the lesser Romulans helmets, which were actually redressed Roman helmets from some of the studio's Biblical epics of the 1950s.
The plot of this episode is based on The Enemy Below (1957), with the Enterprise taking the part of the American destroyer and the Bird-of-Prey with its cloaking device taking the part of the submarine. The episode was also partly inspired another World War II Submarine Drama Run Silent Run Deep (1958), which was directed by Robert Wise, who would later direct Star Trek: The Motion Picture (1979).
Network restrictions at the time forbade the tackling of any contentious subjects such as the Vietnam War, the Civil Rights movement and the rise of feminism. "Star Trek," under the guise of science fiction, boldly flouted these rules. This story, for example, openly deals with the subject of racism as reflected through Lieutenant Stiles' opposition to Mr Spock.
Mark Lenard said, "The Romulan Commander was one of the best roles I ever had on TV". Comparing the part with that of Sarek, Lenard elaborated, "In many ways, I did enjoy that role [Sarek], but I think the more demanding role and the better acting role was the Romulan Commander".
The first time in Star Trek lore to have the ship's captain perform a marriage ceremony for his crew. Although 20th century naval captains were prohibited from doing this, the writers may have gotten this idea from The African Queen (1951). (see the Goofs section and Star Trek: The Next Generation: Data's Day (1991))
Although phasers are used throughout the episode, the visual effect seen is that which later became used for photon torpedo launches, probably because the term "photon torpedo" was not invented until later in the season, in "Arena". Kirk orders the phasers to be "set for proximity blast"; each phaser blast acts like a Navy depth charge.
An almost unnoticed bit of staging might indicate that Angela Martine was Catholic, as she is seen genuflecting before the altar in the ship's chapel during her aborted wedding ceremony. While hardly controversial today, for the 1960s it was a fairly bold thing to show, considering the prejudice against Catholics that was still common in the United States in those days. (Note, however, that this is not exclusively a Roman Catholic practice. Some Episcopalians, for example, also genuflect in front of the altar.)
The Romulan helmets shown aboard the Romulan bridge were designed to cover the ears of the actors. This saved the additional cost of creating prosthetic ear-points for each of the supporting actors. They were reused in Star Trek: Amok Time (1967) (on Vulcans) and Star Trek: The Enterprise Incident (1968) for the same reason.
This is the only time in which the ship's weaponry is fired through a chain of commands (Kirk, to Stiles, to phaser room), although Star Trek: The Corbomite Maneuver (1966) comes close with Bailey's phaser drills. This gives the episode more of its "submarine versus destroyer" feel.
In the James Blish adaptation of this story, presumably based on an earlier draft of the script, Stiles dies. In addition, Robert Tomlinson and Angela Martine actually marry, in a second ceremony late in the story. When the Enterprise fires on the Romulan ship for the final time, the latter explodes immediately, with the conversation between Kirk and the Romulan commander being omitted.
The Romulan Empire is noticeably modelled after the Roman Empire. The terms Centurion and Praetor are borrowed from Ancient Rome. The only named Romulan in this story introduced himself as Decius, which is a Latin name used in Ancient Rome. Despite the heavy Roman theme, he is the only Romulan character throughout the entire run of the original franchise (1966-2002) to have a Latin or Roman name. This would not happen again until Star Trek (2009) with the Romulan character Nero.
Several sources state that the ship's chapel was a redress of the transporter room. Yet in this episode, and the other episode the chapel is seen (Star Trek: The Tholian Web (1968)) the chapel is an obvious redress of the briefing room.
The trivia items below may give away important plot points.
In the original script, Hansen warns before he dies that the Romulans used stolen starship designs to build their ship. This is why Stiles believes there are spies on the Enterprise. The part was deleted from the final print.
In the William Shatner novel The Return, where Kirk is resurrected by a Borg/Romulan alliance to kill Captain Jean-Luc Picard, the Romulan alliance is arranged by the granddaughter of the Romulan commander of this mission, convinced that Starfleet are brutal murderers and seeking revenge for her grandfather's death at Kirk's hands.