Heather Young, who played stewardess Betty Hamilton, was pregnant for much of the show's second season, resulting in her either being filmed mostly from the waist up, wearing baggy outfits or being written out of several episodes altogether.
Often dismissed by science-fiction "purists" such as 'Harlan Ellison' (I) due to the sheer physical impossibility of such "giants" existing. Purists believed the giants' mass (which would increase geometrically while their strength only increased proportionately) would crush them. Murray Leinster's novelization of the show attempted to account for this, but no such explanation is ever given in the series.
In one of the last episodes of the series ("Wild Journey"), Steve and Dan travel back through space and time to before they left Earth, and they attempt to change history by persuading any one of their passengers not to take the fateful flight. Actor Stefan Arngrim, who played Barry Lockridge, had entered puberty and could not reprise his younger self, so a body double (with face obscured) was used.
Deanna Lund stated in an interview that if the show had returned for a third season, the producers were going to explore the possibility of a romance between her character Valerie Scott and Mark Wilson, who was played by Don Matheson. This would have reflected the real-life romance between the two, who married after the show's cancellation.
Although it is supposed to be another planet, nameplates visible on cars and trucks on the show clearly indicate they were manufactured by Chrysler Corporation (the automaker contracted with the series' studio at that time).
Although the series ran for two years (1968-70), the first 12 episodes were filmed in the summer and fall of 1967 to meet the original planned start of the series, which would have been January 1968. The network was originally planning to use "Giants" as a mid-season replacement show. Since nothing needed replacing on their schedule, the series' premiere date was pushed back to the fall of 1968, where it replaced Irwin Allen's Voyage to the Bottom of the Sea (1964) series on the schedule. The 1967 start of the series' filming helps explain the difference in appearance of Stefan Arngrim from year one to year two, because in actuality THREE years have transpired over the course of the series' 51 episodes.