The Killer and Seven collaborators David Fincher and Andrew Kevin Walker dissect their lethal partnership

"I never say never."

Generally good words to live by. In the case of director David Fincher, they're also good words to work by, at least as it pertains to entertaining the notion of making a sequel to his new film, The Killer, available now on Netflix. "It doesn't pay to have rules with that stuff. I'm the guy who, before Zodiac, said, 'No more serial killers.'"

The joke is typical Fincher: dry, winking, and only humorous to those who possess the proper context. The filmmaker who brought us Kevin Spacey's serial killer John Doe in 1995's Seven would, of course, continue to explore similarly murderous terrain, not just with 2007's Zodiac, but in 2011's The Girl With the Dragon Tattoo, and on two seasons of his Netflix show Mindhunters. While The Killer is not a serial killer film, it certainly has a series of killings. The movie stars Michael Fassbender as a nameless hit man who, after a job goes wrong, sets about visiting with routinely lethal consequences a succession of folks — including two fellow assassins, one played by Tilda Swinton — who might pose a threat to his future.

The film reunites Fincher with Seven screenwriter Andrew Kevin Walker in the pair's first credited big-screen collaboration since the Brad Pitt-starring hit and pop culture sensation. The director tasked Walker to come up with a script that kept the dialog of Fassbender's central character to the bare minimum. Walker recalls that Fincher told him to, "try and write it so this guy has literally ten lines of dialog spoken in the entire movie. As a point of pride, I did hand in a first draft that had literally 13 lines of dialog. It was the most I could get it down to and still have it function and be semi-natural."

Fassbender's character is voluble in voiceover as he mentally recites a mantra that extols the virtues of a strict and dispassionate approach to his job even as his actions become ever more improvisational and emotionally engaged. "I'm not here to take sides," Fassbender stoically intones as his Killer prepares to take a shot at his target in the film's lengthy, Paris-set opening sequence. "I serve no god or country. I fly no flag. If I'm effective, it's because of one simple fact: I. Don't. Give. A. F---." Screenwriter Walker recalls that "from the very beginning, one of the big things that David said was, 'He's going to have this mantra that he repeats and then, as he goes, the mantra will begin to kind of crumble, or he'll contradict it. And no matter how unemotional he says he is, it will all be driven by a deep, deep, deep-seated emotion that he doesn't even want to admit is there.'"

The work of crafting that voiceover began long before shooting and continued deep into post-production as Walker and the legendarily meticulous Fincher sought just the right combination of words.

"We started to go at it, and we kept at it, and we kept at it," says Walker, whose non-Fincher credits include 1999's Sleepy Hollow and last year's Jason Segel-starring thriller Windfall. "There may have been times — and I'm not saying this was an argument — but there may have been times when I was going, eh, feels about right to me, I'm done, baby! But it was David's insistence that we keep discussing it. I'm really grateful on his insistence to keep finding the balance, because some of the stuff that came very late in the process of discussing and adjusting voiceover are not only some of my favorite things in this movie but some of my favorite things I've ever written."

Fincher himself calls working on the voiceover as "an interesting balancing act and, yeah, invariably there will come a time when you go, okay, I need you to express all of the thoughts in these 12 words and you have four. And I don't know how you're going to get there either and don't bug me!" he concludes, with a laugh. (David Fincher laughs a lot more in conversation than you might imagine.)

Walker cannot have been surprised by Fincher's diligence. While the Pennsylvania native has not been a credited writer on any of the director's movies in the almost three decades since the release of Seven, he has worked with Fincher on many projects, including 1997's The Game and 1999's Fight Club. "I'm very proud of my uncredited work," he says. "I did do a polish, strictly a polish, for Fight Club. I did a bunch of rewriting, much more extensive on The Game, which was another incredible pleasure to be involved in." Additionally, Walker has worked on screenplays for proposed Fincher projects that have yet to be made, notably adaptations of Max Ehrlich's horror novel The Reincarnation of Peter Proud and Jules Verne's science fiction classic Twenty Thousand Leagues Under the Sea, which the filmmaker was going to direct for Disney. Walker says the latter movie "was very close to getting made. That would have been the coolest Twenty Thousand Leagues Under the Sea you could imagine. It was going to be a movie where parents would be as blown away as the kids."

Walker clearly has a great fondness for Fincher, whom he regards as a friend and something of a soulmate.

"There was a time when I was working with David on The Game, and it's kind of silly to mention it, but it always just makes me laugh," the writer recalls. "He was saying, 'What's that song? What's that song?' And I was like, 'Oh, you mean 'Cat's in the Cradle'? I like to say, we finish each other's sentences, but then Dave very gently edits that sentence down."

When EW later tells Fincher how complimentary Walker has been about him the director chuckles, calling the writer "a f---ing liar." The director then goes on to explain that there is "a kind of solace that one gets to experience when Andy's on the case. When you say, in terms of the plotting, 'Why do this and not this or this,' a lot of people will say, 'Let me get back to you.' Andy's going to take out his composition book which has all of his John Doe writings in it, and he's going to flip to the page, and he's going to go, yeah, I gamed that out. There aren't many minds like that. This is a guy who can take you through a half-hour discussion as to why something needs to land in the first quarter of page 23. [Laughs] It's nice to turn over that aspect of the laying out to somebody who takes it so seriously and engages with it so thoroughly."

Walker definitely seems to have earned his money on The Killer, which is based on a French comic book series, and not just because of his willingness to rewrite the voiceover countless times. For example, it was the screenwriter who suggested one of the hit men visited by Fassbender's character be a hit woman.

"From the very beginning, when we talked about Tilda's character, we talked about 'him,'" says Walker. "I had a discussion with David and said, 'But what if it's a woman? You've worked with Tilda (on The Curious Case of Benjamin Button), you can ask her to do this. She would be the ultimate person to do it. Hey, Dave, give her a call!' And god bless her, she said 'Yes.' Some of the Tilda stuff, I'm just so proud of, but you just can't help but be proud of it, because Tilda found it acceptable to utter it, and to imbue it with such grace and amazing life."

It was also Walker who came up with the film's running gag of the Killer's aliases being the names of characters from old sitcoms: Sam Malone from Cheers, Felix Unger from The Odd Couple, and so forth. The writer explains that he realized there would be times when the Killer would have to use a name and that Walker "didn't want it to be the same name, because, even if that's his alias, it becomes his name. So it just made sense to shuffle the deck of names we used every time. Fincher, to his credit, and in his genius, made sure to really hit them. In my draft, they were just spoken last names; in the movie, he really makes sure you see and hear not just last names but full names. It's one of my favorite notions about this film that there'll be some 20-year-olds sitting beside some 50-year-olds, [and] the 20-year-old is wondering, why in the hell is this person laughing every 10 or 15 minutes?"

So what's next for the dastardly duo? Walker teases that the pair has a certain unmade project that he would love to see resurrected ("Hint! Hint!"). He also says that he wants to "write a Christmas movie. Probably with a serial killer."

"Sounds kind of cool," says Fincher, when EW relays this information to the director. He is joking, of course.

But, then again, never say never.

The Killer is now available to stream on Netflix. Watch the film's trailer below.

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