This is the first time that live two-way communication with Starfleet Command is depicted. In previous episodes, communication with Starfleet Command was through delayed radio messages.
After John Drew Barrymore failed to show up on the set, director Gerd Oswald decided to shoot scenes which didn't involve him. On the second day, it was decided to either shut down production and scrap the episode overall or find a replacement. Robert Brown was dragged in to the set right after he agreed to play the role. He recounted the filming to be very tight and tense.
John Drew Barrymore was originally cast as Lazarus, but failed to show up for shooting and had to be replaced by Robert Brown causing the episode to go two days over schedule. Star Trek's producers subsequently filed and won a grievance with the Screen Actors Guild, which suspended Barrymore's SAG membership for 6 months.
In the original script, Lazarus romantically came on to Lt. Charlene Masters to gain her assistance. The two subsequently fell in love, but when African-American actress Janet MacLachlan was cast as Masters the romantic angle was dropped. In addition, Gene Roddenberry considered it too similar to the romance between Khan and McGivers in Space Seed (1967). As stated by Roddenberry in a Season One memo: "In both 'Space Seed' and this story, we have a crew woman madly in love with a brawny guest star and flipping our whole gang into a real mess because she is in love... do they have to do [this] in two of our scripts?"
Along with Friday's Child (1967), this is one of the only two episodes where outdoor planet scenes were filmed both on Desilu Stage 10 and on location (both times at Vasquez Rocks). Originally, all planet-side scenes were scheduled to be filmed on location, but due to the turmoil during production director Gerd Oswald couldn't finish shooting at Vasquez. Matt Jefferies and the art department prepared a spot on Stage 10 which could accomodate the missing "alternate universe" sequence.