The Cast Showoff: Ryu Manatsu (Gen) sings the first theme song, does the awesome martial arts/gymnastics demonstration in the very first scene, as well as sings a song in which Gen yearns for his home planet.
According to the Ultra Wiki, Ryu Manatsu did nearly all his own stunts, except in Episode 3 (some of the fights vs human-sized Tsuruk) and Episode 8 (possibly somebody else did the Parkour?).
What Could Have Been: When Tsuburaya Productions cast Kohji Moritsugu as captain of MAC, his character was meant to be a new character named Tetsutaro Kawakami. However, Kohji Moritsugu was concerned viewers would confuse Kawakami with Dan, so the character was changed, which probably explains the complete change of character.
You Look Familiar: Kaoru Sugita played a supporting role in Episode 31 before she reappeared in Episode 40, playing Azumi Miyama, Nurse Miyama's youngest daughter.