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In June 2018, Danny McBride confirmed that he and David Gordon Green had originally intended to pitch two films that would be shot back-to-back, and then decided against it, waiting to see the reaction to the first film:
"We were going to shoot two of them back-to-back. Then we were like, 'Well, let's not get ahead of ourselves. This could come out, and everyone could hate us, and we'd never work again. So, let's not have to sit around for a year while we wait for another movie to come out that we know people aren't going to like.' So, we were like, 'Let's learn from this, and see what works, and what doesn't.' But we definitely have an idea of where we would go [with] this branch of the story and hopefully we get a chance to do it."
Jamie Lee Curtis, Charles Cyphers, Kyle Richards, Nancy Stephens, and Nick Castle were all in the original Halloween and all returned for this sequel. The characters of Tommy Doyle and Lonnie Elam from the original also returned, but were recast with Anthony Michael Hall and Robert Longstreet.
Dr. Loomis, played by Donald Pleasence in Halloween (1978) and four of its sequels, surprisingly wasn't brought back through special effects or VFX trickery. He was played by set constructor Tom Jones Jr. in his very first film role; with the help of practical make-up appliances, Jones, Jr. bears an uncanny resemblance to the late Pleasence. He was then dubbed over by Colin Mahan, who previously voiced the good doctor in an audio recording during Halloween (2018). Careful lighting, editing and digitally adding grain to the footage completes the illusion that these scenes were shot in 1978. Jones, Jr. was originally supposed to appear as Loomis in an unused opening scene for Halloween (2018) as well, which would have retconned the ending of Halloween (1978) and have Michael Myers kill Dr. Loomis, after which Laurie Strode shoots Michael. However, the makers ultimately chose not to go through with this, and continue the original ending of Halloween (1978) in this movie.
Following the partial reboot Halloween (2018), this film is part of the fifth separate Halloween timeline: Original intent timeline (John Carpenter acknowledging that the original 1978 movie was always intended to be one complete story): Halloween (1978) Original timeline: Halloween (1978), Halloween II (1981), Halloween 4: The Return of Michael Myers (1988), Halloween 5: The Revenge of Michael Myers (1989) and Halloween: The Curse of Michael Myers (1995). Halloween III: Season of the Witch (1982) (completely separate continuity). Halloween (1978), Halloween II (1981), Halloween H20: 20 Years Later (1998) and Halloween: Resurrection (2002) (which ignore parts 3 through 6 of the original timeline). Halloween (2007) and Halloween II (2009) (complete series reboot). Halloween (1978), Halloween (2018) and Halloween Kills (2021), followed by Halloween Ends (2022) (which ignore part 2 through 6 of the original timeline).
On September 5, 2019, it was announced that indie cinema and festival darling Robert Longstreet has joined the cast as adult Lonnie Elam, who bullied Tommy and tried to make him believe that the boogeyman was coming after him in the 1978 original. Lonnie is also the same kid who was dared to go into Michael Myers' old house, and while hiding in the bushes, Dr. Loomis famously yelled, "Hey! Hey, Lonnie, get your ass away from there!" to which Lonnie and his friends scrammed.
Malek Akkad addressed and explained why it would have been distracting for Paul Rudd to reprise the role of Tommy Doyle (whom he played in Halloween: The Curse of Michael Myers (1995)). At a press event attended by Screen Rant, Akkad admitted it would have been "wonderful" had Rudd reprised the role in Halloween Kills, adding, "It would have warmed my heart and definitely there's some disappointment there." Ultimately, however, Akkad felt Anthony Michael Hall "fulfills this role in a way that's maybe less jarring for the narrative and maybe won't take people out of it." He added, "Because Paul Rudd is so big for a lot of things like comedy and whatever, he's a fantastic actor, I think it turned out this way for a reason and we're so excited. Anthony Michael Hall does a knockout performance, it's definitely gonna be one of the fans' favorites."
Busta Rhymes, who was in Halloween: Resurrection (2002), was almost included in the film but was scrapped.
According to Andi Matichak, it was initially planned that Halloween Kills and Halloween Ends (2022) would be filmed back to back. This did not occur, due to an "intense schedule."