Natascha McElhone was originally set to voice Cortana, until Jen Taylor, who voices the character in the Halo video game series, replaced her. McElhone plays the role of Dr. Halsey only.
Pablo Schreiber said his Master Chief armor weighed 50 pounds. Bentley Kalu reported that his armor for Vannak-134 weighted roughly 30 kilograms (66 pounds). In the main Halo canon, the Mjolnir battlesuit weighs 1,000 pounds when in use.
To prepare for the show, many of the actors received different forms of Halo media to study. Pablo Schreiber, who plays the Master Chief, watched every Halo cutscene from the games, watched every live action movie and animated short, as well as read several of the books and graphic novels. Kate Kennedy, who plays Kai-125, beat the campaign for each main Halo game and read The Fall of Reach. This comes from Schreiber and Kennedy's interview with Paramount+.
All the weapons in the show are accurate in scale to how they are in the games and books. With an exception being to Kai-125's sniper rifle. Kate Kennedy told Paramount+, that since the rifle is written as being the height of a Spartan, it was adjusted by 7 inches to match her height of 6 feet 3 inches.
Filmed in Hungary, mostly in Korda Studios in Etyek, near Budapest.