Casting
In September 2017, Jamie Lee Curtis confirmed that she would reprise her role as Laurie Strode. In contrast to the character's final girl role in the original film, Laurie armed herself and prepared extensively in the time period between films in case Michael Myers returned. Although Halloween II and its later installments have portrayed Myers as a familicidal killer and Laurie as his sister, the writers felt that the added motive made him less frightening as a killer. As such, they intentionally ignored that aspect of the lore. In the film, Laurie's granddaughter Allyson explains how her life has been impacted by Michael's homicidal rampage 40 years earlier. When a friend hints that he had heard Michael was Laurie's brother, Allyson replies, "No, it was not her brother. That was something people made up." The writers did not originally know if Curtis would be willing to return, according to McBride, so they "busted [their] ass on this script to really make that Laurie Strode character something she wouldn't be able to say no to." On why she returned, Curtis stated, "As soon as I read what David Green and Danny McBride had come up with [...] and the way that they connected the dots of the story, it made so much sense to me that it felt totally appropriate for me to return to Haddonfield, Ill., for another 40th-anniversary retelling. It's the original story in many, many, many ways. Just retold 40 years later with my granddaughter." Curtis had previously returned as Laurie in the sequels Halloween II (1981), Halloween H20: 20 Years Later (1998), and Halloween: Resurrection (2002). The following October, Judy Greer entered negotiations to play Laurie's daughter, Karen Nelson. On December 7, 2017, Andi Matichak was cast to play Laurie's granddaughter Allyson.
Danielle Harris, who played Jamie Lloyd in the original continuity's Halloween 4 and Halloween 5, contacted Blumhouse with the offer to reprise her role in some way, but the studio went with a different daughter character. Harris and certain horror publications expressed their disappointment: "I was okay with it when she had a son [...] but they're saying it's the last one and [...] she has a daughter. And it's not Jamie. It's just kind of a bummer, I guess."
On December 20, 2017, it was announced that Nick Castle, who had portrayed Michael Myers in the original film, would reprise his role, with actor and stuntman James Jude Courtney set to portray Myers as well. Courtney was suggested to Malek Akkad and David Gordon Green by stunt coordinator Rawn Hutchinson for his ability to do both physical stunts and genuine acting, auditioning afterwards and receiving a phone call in December 2017 affirming that he had landed the role. Green explained to him his vision for Myers's mannerisms, an amalgamation of Castle's original performance and the addition of an efficient cat-like style of movement. Courtney tailored his portrayal to those specifications from observing an actual cat, "I think cats are the most perfect hunting machines on the planet. And the beauty of it is we don't judge a cat for what a cat does. So I sort of carried that movement and the non-judgmental approach to the way I moved as The Shape, which I learned from my cat Parcival." He referred to collaborating with Castle as an "honor", while Castle described it as a "passing of the torch". Courtney used John Carpenter and Castle's work on the original film to determine how the 40 years that transpired between the events of the films would inform the character over time. The December 2017 announcement of Castle's participation was widely reported as his retaking the role of Myers he originated, with Courtney only doing additional work as the character.
On January 13, 2018, Virginia Gardner, Miles Robbins, Dylan Arnold and Drew Scheid were confirmed to play Allyson's friends. On January 16, 2018, Will Patton was publicized to have joined the film's roster. He was later joined by Rob Niter, both actors being announced to portray police officers, as well as British actress Rhian Rees, who was cast as a character named Dana. Speaking of the cast, Castle stated that "What I like about this (new film) is they've got some really good young actors. They fleshed out the relationship of Jamie's character with her daughter and her granddaughter. And they made some choices that I think are really bold choices about who these people are and why they are the way they are now."
On July 27, 2018, it was announced that a sound-alike actor would provide a voice-over for Dr. Sam Loomis, who was originally portrayed by Donald Pleasence in the original film, Halloween II, Halloween 4: The Return of Michael Myers (1988), Halloween 5: The Revenge of Michael Myers (1989), Halloween: The Curse of Michael Myers (1995), before his death in 1995. In the film, the voice of Dr. Loomis is heard giving advice that Michael be executed, claiming that Michael needed to be killed because there was no point in keeping evil alive. Loomis is featured in a vocal cameo when Aaron and Dana listen to a recording of Loomis, made three months after the events of the original film. Loomis is voiced by sound-alike stand-up comedian Colin Mahan, who was used to do Donald Pleasence impressions. Mahan was hired after one of the film's executive producers heard him by chance and suggested him for the part. At Carpenter's suggestion, Green, McBride and Fradley initially wrote some drafts where Loomis was more involved in the story, appearing onscreen at the beginning of the film, with the trio devising to cast their art director Sean White as Loomis due to his look-alike resemblance to Pleasence, but the idea went unused for it. The sequel, Halloween Kills (2021), ended up incorporating this concept, but with Tom Jones Jr. instead for that film's 1978 flashbacks. Additionally, P. J. Soles, who had portrayed Lynda van der Klok, Michael's final victim in the original film, had cameo appearance as a teacher.
