Anachronic Order: Was originally filmed to serve as a mid-season break, capping the first thirteen episodes and creating a shift into the second set of thirteen episodes that would involve Number Six working outside of the Village in various acts of espionage. When it became clear the series wasn't going to continue for another thirteen episodes, they held off on this episode and created a Grand Finale episode "Fall Out" to exactly follow this one.
Author Avatar: MacGoohan wrote the screenplay, and various scenes were based on actual incidents from his childhood.
Brainwashed and Crazy: During the Degree Absolute setup, Number Six is brainwashed into refusing the concept of a number six, forced instead to focus on "number five." He's also mentally reduced to the capacity of a two-year-old, from which Number Two could begin the seven stages of manipulating Number Six into breaking.
Darker and Edgier: The entire episode is grim and moody, with the main characters Number Two and Number Six on edge and angry. It gets edgier when they engage inside Degree Absolute, and darker when Number Six succeeds in killing Number Two during the final countdown.
Hoist by His Own Petard: Number Two tries to use intense psychological acting-out to force Number Six to break. Unfortunately, the process allows for the roles of interrogator and prisoner to switch from time to time... and they unfortunately switch roles during the last phase when death is the endgame.
It didn't help that Number Two admits at the end he had his own psychological issues, which sucked him into the Degree Absolute process too well.
One nursery rhyme that Number Six begins singing during a regression phase is "Pop Goes the Weasel." Number Two panics when he hears it and openly questions if "Pop" refers to "Protect Other People."
As Degree Absolute counts down to zero, Number Two begins reciting the numbers: Ten... Nine... Eight... Seven... Six... When he reaches "six", Number Six begins shouting "Die, Six! DIE!" and answers each remaining number with "DIE" until Number Two reaches "Zero..." and he dies.
Shout-Out to Shakespeare: Number Two refers to the "Seven Stages of Man" that Shakespeare writes about in As You Like It when describing the Degree Absolute process to a brainwashed Number Six: the process forces the victim to live out the seven stages - child, schoolboy, lover, soldier, justice, wise elder, finally senility(death) - during which Number Two hopes to reveal Number Six's secret or at least break him into serving the Village.
Troubled Production: The episode was written in a day to meet a grueling deadline. The intensity of the performances was such that actor Leo McKern suffered either a heart attack or a mental breakdown soon after filming wrapped up.
Villainous Breakdown: Number Two begins the episode in a foul mood and proceeds to get too involved inside the Degree Absolute program. It leads to his death.
The intensity of the performance was enough to drive McKern to either a heart attack or a nervous breakdown (maybe both).
Wham Episode: Number Two forces Number Six into a duel of wits and psyche, a process which kills Number Two. The Butler switches side to Number Six because "he's boss now." The Supervisor shows up to escort Number Six and the Butler to meet Number One, and the episode goes to black as the sound of a nuclear missile launch echoes in the background...
Worthy Opponent: How Number Two sets up the battle of wits. He realizes a one-on-one fight with Number Six was the only fight Number Six would respect and engage. Hence going for a step of last resort like Degree Absolute.