Although it might be intimidating for a director to hop aboard a successful television series a few seasons in, didn't have that problem on Slow Horses. "The speed with which the show is filmed meant that only season one was out when I got the job," he told Gold Derby. "Season 2 was coming out about two weeks later, and Season 3 was when I was already in post [production]. So it hadn't reached the level of success that it was going to [have], but it would still have made a big splash" given its high pedigree and A-list cast. "I was unbelievably excited and definitely intimidated going in."

Based solely on what he had seen and read, it was clear to Randall that "each season was very different" and evolving. "They've established a world, they've established these characters, but each season's got its own sort of flavor, plays with genres in different ways, has its own tone. So that was one of the really exciting things, being able to take something from this remarkable world and put a stamp on it," Randall said. And in this case, "We definitely felt that it was a darker season," and every decision he made branched off from that.

Season 4 of Slow Horses was "heavier emotionally, and in a way, it was the first season that was genuinely emotional," considering it dealt with, among other things, retired MI5 officer David Cartwright's () decline into dementia and the effect it has on his grandson, River (). Combined with the inciting terrorist attack in Episode 1 and Frank's () "bizarre" story set at a French compound "allowed for a much darker, heavier visual palette and score." There were even "elements of horror in there" in its portrayal of David's decline, "as well as the thriller elements," but also "the comedy and the drama that people love" from previous seasons.

Slow Horses was Randall's first foray into television, as he'd previously only worked in films. Considering he directed all six episodes, "it was a very easy transition, because in effect these are six-hour movies." That was mandated from the beginning by , who stars as crotchety MI5 agent Jackson Lamb. "That was one of the things that he was adamant about, that he wanted one director for all six [episodes] and to treat them like a movie." Because of this, "the show was allowed to be very director-led," Randall explains, "so once I figured out the lay of the land," he realized it was "very similar to a film, and I could push it in certain directions and think about it from start to finish."

He gives due credit to lead writer , who is "the loveliest man on earth, as well as incredibly talented and very, very funny." The two had actually met years prior because, Randall reveals, "he was in a short film I'd directed a long time ago that no one should ever see," even though, "Will was good in it, and it was lovely meeting him through it." Although the scripts Smith had written "were already great," Randall was able to bring in his own "had thoughts and ideas," especially when it came to the action sequences. "They just have to evolve and change, and he was always really open and allowed me a lot of freedom."

SEE Gary Oldman on 'Slow Horses' being 'an extraordinary show to work on' and 'one of the highlights of my career'

That collaborative spirit extended to his relationship with the cast. "It's interesting with this, because obviously most of them have done three seasons, so in some ways I was catching up," Randall divulges. "I was asking them about their characters," and also "what they wanted to achieve in season four." Although he'd rehearse certain scenes before filming, his directorial technique "really depends on the actor. Some actors like to keep it very fresh on the day, some actors like to talk in a lot of detail about everything, from the props they may be holding to their intention in a scene." When they arrived on set, "we'd just be very loose and play," coming up with new ideas on the day.

"The one thing with those actors is how remarkable they already are," Randall continues. Whereas in "some of my previous work" he had to "try to find the performance," in Slow Horses, "it was already of such a standard that had I have not turned up it still would've been a great scene. So you're not trying to find the performance, you're trying to find what else can we bring? What other nuances can we bring? How can we play? Can we bring out any humor here? Is there any more we can push it towards?" It helped that "we also had a lot of new cast" who brought "different energies," which allowed the veteran stars "to do very different things this season."

Randall submitted the fourth-season final for Emmy consideration, which he admits "was a hard decision." He chose the episode because, "I thought it balanced a lot of the different things that I was really proud of." For one thing, "it was the most emotional episode" for all of the characters, especially River and David. There was also "a lot of action, which we were really proud of, and we were bold in how we did it." The finale "was a way of weaving all of these different elements together, telling all of these different character's stories, holding this emotion in a way" that still feels in keeping with the rest of the series. It was "a huge challenge," yet the result was "an hour of drama that I'm really proud of."

As Season 5 is set to premiere in September, Randall is already in post-production on Season 6, although true to the art of spy-craft, he's keeping tight-lipped about any details. "I definitely can't share details, other than I'm really, really proud of it," he reveals. "It takes this show to new places. It's a big, dramatic, and emotional season, and I'm very excited about it."

Slow Horses is streaming on Apple TV+.