Writing
Before screenwriters were hired, Miller requested that a group of novelists be consulted on how to reinvent the franchise.[61] Among the novelists were Joe Abercrombie, Neal Asher, Greg Bear, Warren Ellis, and Neal Stephenson.[44] Abercrombie suggested the idea of a female character who is half human and half machine, forming the origins of the character Grace.[61] A human-machine character was previously featured in Terminator Salvation.[62]
The film's story was conceived by Miller, Cameron, and Ellison, and a team of writers was hired to write the script. The team included Charles H. Eglee, David S. Goyer and his writing partner Justin Rhodes, and Josh Friedman, creator of the television series Terminator: The Sarah Connor Chronicles.[49][63][64][44] Cameron and the writers watched the Terminator sequels that came after his initial films, and it was determined that the storylines of the later films were too complex when it came to time travel.[49][50] Weeks were spent working on the story, which was eventually envisioned as a new Terminator film trilogy.[49][65][66] Goyer wrote a draft for the first film in the trilogy that would ultimately become Terminator: Dark Fate.[44]
Goyer moved on to other projects,[44] and by November 2017, Billy Ray was brought in to polish the script.[63] Ray rewrote much of Goyer's draft. Miller wrote the film's action scenes, while Ray handled the characters.[44] Cameron had a list of action scenes, for no particular film, that he had wanted to shoot over the years, and he gave this list to Miller so he could work them into Terminator: Dark Fate. The list formed the basis for scenes involving a dam and a Humvee underwater.[67] As the start of filming approached, Cameron felt that the script needed improvement and made the changes himself.[49] The film's story credits were given to James Cameron, Charles Eglee, Josh Friedman, David Goyer, and Justin Rhodes; screenplay by David Goyer, Justin Rhodes, and Billy Ray.[68] Cameron said that he and Miller ultimately had many disagreements about the film, but he described it as being part of the creative process.[46]
Miller said that the destruction of Cyberdyne at the end of Terminator 2: Judgment Day is an event which would change the future "but no one knew how. And I don't think the movies that came after it really explored that in a clean way like I believe we are, with true consequences, and it makes perfect sense for Sarah to be the one to face those consequences since they were her choices to begin with."[69] One consequence would be the death of John Connor, who was initially meant to become the future leader of the human resistance against machines.[67] The decision to kill the John Connor character came from Cameron, who wanted to surprise audiences who had become invested in the character's mythology: "It's like, 'Let's just get that right off the table. Let's just pull the carpet out from underneath all of our assumptions of what a Terminator movie is going to be about. Let's just put a bullet in his head at a pizzeria in the first 45 seconds.'"[70] Cameron said that John's death serves as "a springboard for the story to show Sarah's ultimate trauma from which she only begins to recover right at the end of the new film. She's driven by hatred, by revenge. ... Her badassery comes from a place of deep hurt and deep pain."[70]
Miller said the decision to kill John was not controversial among him and the other filmmakers. Miller felt that Sarah Connor was best portrayed as an unhappy character, and he said that John's death provided a reason for her to be that way.[67] Miller said about Sarah Connor, "Grief has made her want to be an emotionless killing machine. And at the end of the movie, she's allowing herself to care again, she comes back to humanity. Her shriveled heart has blossomed again. That was the journey". However, Miller did not want Sarah Connor to be an unpleasant and "unwatchable" character, and said, "I think Sarah is tough, but it's not uncomfortable to watch."[44]
Cameron believed that removing John Connor would prevent the film from feeling like a retread of previous films.[71] Discarding John Connor allowed for new characters to be worked into the story. Additionally, Miller said, "You can't have John be a 36-year-old accountant somewhere. And really, when you think about it, he could be sort of a pathetic figure as a man who had missed his moment in history and was relegated to this banal, ordinary existence". Describing the opening scene, Miller said, "You want to slap the audience in the face and say, 'Wake up. This is going to be different.' I feel like that accomplished that. I hate the violence of it. I hate the idea of a kid being shot, but the dramatic fuel that it gives the story is kind of undeniable." In the early stages of development, there was consideration given to the idea that Dani Ramos could be portrayed as John's daughter, or that she could have some other connection to the Connors. However, Miller disliked the idea that she would be related to them.[67] There were never plans to feature John Connor in any other scenes besides the opening.[70] Linda Hamilton was somewhat shocked by the decision to kill John Connor, but she also said she wanted the film series and its characters to evolve.[71] She was pleased with the film's characters, and felt that earlier sequels to Terminator 2 lacked characters that the audience would care about.[72]
Miller was dissatisfied with the final film's idea that Dani would send Grace to the past, saying, "We set up this whole [story] where Grace is kind of Dani's surrogate child and a mother sending her child to die for her is just...yeah, I had a different scene in mind."[73] Additionally, several different endings were considered, including one in which Sarah and Dani would bury Grace, and another in which Grace's body would be burned and sent down a river. Eventually, Miller suggested the idea that Dani would go to see the younger Grace. The ending playground scene was a late addition to the film.[73]
Cameron devised the idea of a T-800 Terminator that is "just out there in this kind of limbo" for more than 20 years after carrying out an order, becoming more human "in the sense that he's evaluating the moral consequences of things that he did, that he was ordered to do back in his early days, and really kind of developing a consciousness and a conscience". Cameron considered this iteration of the character to be more interesting than those featured in his first two films, saying, "We've seen the Terminator that was programmed to be bad; you've seen the one that was programmed to be good, to be a protector. But in both cases, neither one of them have free will."[46] Schwarzenegger enjoys interior decorating, so Cameron suggested that his T-800 character in the film have a drapery business.[74][75][76]
Casting
By April 2017, Schwarzenegger had joined the project to reprise his role.[77][78][79] In September of the same year, it was announced Hamilton would return to reprise her role as Sarah Connor.[80] Hamilton last portrayed the character onscreen in Terminator 2, although she also provided her voice in an uncredited role for Terminator Salvation.[81] Because previous Terminator films did not do well with audiences, Miller felt it was necessary to have Hamilton reprise the role.[69][82] Cameron, Ellison and Miller only wanted to bring back the Sarah Connor character if Hamilton would reprise the role. The film's storyline was devised first so the trio would have an idea to pitch to Hamilton.[49] Cameron said that he sent Hamilton a "long rambling email with a lot of reasons why she should do it and a lot of reasons why she shouldn't." Cameron's main reason for why Hamilton should return was that people liked her in the role.[48] There was never a version of the film that excluded Hamilton, and Miller said there was no backup plan in the event that she declined the role.[83]
After approximately six weeks,[84] Hamilton chose to sign on to the film,[69] which did not yet have a completed script for her to read as it was still being refined.[85] Initially, Hamilton was not sure if she wanted to reprise the role.[86] She had been semi-retired from acting,[87] and said, "I didn't want it to look like a shameless money grab. I am living this quiet, lovely life that doesn't involve being a celebrity, and you really have to think, do I really want to trade that in again for another 15 minutes?"[84] Because so much time had passed since her 1991 appearance as Sarah Connor, Hamilton had assumed that she would never reprise the role, and she was surprised by the offer to do so.[69] Hamilton said about her decision to return, "I was very pleased that all of the years had passed, because I could fill the years up with so much backstory and inner life that could power the character."[85] Hamilton spent more than a year working with a fitness trainer to get into physical shape for the role.[85][69][88] Hamilton said she put 10 times the effort into her physique than she did for Terminator 2. This included a regimen of supplements and bioidentical hormones, as well as training with Green Berets.[89][90] Commenting on Hamilton's role, Cameron said he liked the idea of an action film starring a 62-year-old actress.[91][92] Hamilton chose to dye her hair gray for the film, as she wanted viewers to see her character as an old woman.[93]
The production was also looking to cast an 18 to 20-year-old woman to be the new centerpiece of the story.[80] Hamilton rehearsed lines with several actresses who were auditioning for the role of Dani, and she immediately felt that Natalia Reyes was the right choice.[94] In March 2018, it was announced that Mackenzie Davis had been cast in the film.[17] Miller said about Davis, "I didn't just want a woman who could physically fit the role but emotionally as well. Mackenzie really wanted to do it; she came after the role. She worked harder than anybody."[61] After Davis was cast, she undertook physical training for the film's fight scenes.[95][96] Schwarzenegger and Gabriel Luna also underwent physical training for the film.[97]
Because the film is partially set in Mexico City, the cast includes several Latino actors,[98][99] including Reyes, Luna, and Diego Boneta, who were cast as primary characters in April 2018.[19] Reyes said, "This movie is a reflection of Hollywood now. We are just changing these stereotypes and the ideas and the cliches of what a Latino should be."[98] By June 2018, Jude Collie had been cast as the double for a young John Connor, with Brett Azar reprising his role from Genisys as the body double for a younger T-800.[27]
Cameron announced in July 2019 that Edward Furlong would reprise his role of John Connor from Terminator 2: Judgment Day.[29] Furlong later said that his role in the film was small,[100] and Miller regretted that Cameron made such an announcement.[101] Furlong's likeness was used to digitally recreate his younger face through CGI, and he also gave a performance through motion capture footage of his face that was added into the film.[70][28][101] Furlong is credited as "John Connor reference".[31]