Leonard Nimoy had not previously appeared on the show because he had set his asking price for such an appearance at $1 million, which was far beyond the budget of the series. However, since this episode was in some ways a "teaser" for the upcoming Star Trek VI: The Undiscovered Country (1991) (on which Nimoy was an executive producer, and entitled to a percentage of the profits of that film), Nimoy agreed to appear in this two-part story for standard SAG pay.
Since this episode was broadcast in November 1991, the month before the release of Star Trek VI: The Undiscovered Country (1991), it could be considered promotion for the motion picture. Spock's dialogue mentioning the movie's overtures of peace with the Klingons and incident at Camp Khitomer, but not the outcomes of either, would support this.
According to Cliff Bole, Leonard Nimoy originally wanted his son, Adam to direct "Unification", but this fell through, so he was called in as director, despite it not fitting into his usual four-episode rotation. Later, Adam Nimoy directed Rascals (1992) and Timescape (1993).
This episode marks the first occurrence of the Romulan greeting "jolan tru", said by the waitress who serves soup to Picard and Data.
This episode actually foreshadows both of Spock's subsequent appearances. He makes reference to his actions in Star Trek VI: The Undiscovered Country (1991), which was a month from release. In the present story, he is working on behalf of the Romulans to reconcile them with the Vulcans. His continuing advocacy for the Romulans leads to him into conflict with Nero (Eric Bana), leading into the events of Star Trek (2009). A comic book mini-series leading up to the events of that film shows that Spock's and Nero's relationship was concurrent with Jean-Luc Picard and his crew's tenure aboard the Enterprise.