‘Slow Horses’ Star Kadiff Kirwan Didn’t Tell His Loved Ones How Season 4 Ends: “I’ve Been Lying For 13 Months To Everyone I Know”

In the 13 months leading up to Slow Horses ‘ highly-anticipated Season 4 finale, actor Kadiff Kirwan was hard at work elaborately lying to his loved ones.

Slow Horses Season 4 finale spoilers ahead.

In hopes of preserving the episode’s shock value and pulling off one heck of a prank, Kirwan — whose character Marcus Longridge dies in Season 4’s final hour — assured family and friends that showrunner Will Smith strayed from Mick Herron’s beloved book series , spared Marcus’ life, and killed off Rosalind Eleazar’s Louisa instead. He fully expects his siblings to freak out when the big reveal unfolds on Wednesday, October 9.

“For the last year, I’ve been telling them I’ve been filming the show. I’ve actually been filming other stuff. They have no idea,” Kirwan giddily told Decider over Zoom ahead of the finale. “When the final episode goes out, we’re going to watch it together, and I’m going to try and sneakily record them watching it. I can’t wait for them to see that.”

Despite Kirwan’s excitement over what’s sure to be a legendary viewing party, he’s admittedly “devastated” to say goodbye to the Emmy-winning Apple TV+ series and the talented cast and crew he’s worked with since Marcus’ Season 2 introduction. That said, if his Slow Horses character had to die, Kirwan is glad he went out with a bang.

“I’m happy that it happened the way it did. If you’re going to die, you want to go down in a blaze of glory,” the actor told Decider. “Am I happy he’s dead? No. Goodness sakes. Mick Herron. Will Smith. Rewrite! But I’m grateful that he got that moment in the sun to try and save his pals.”

In honor of Marcus’ final episode, Kirwan chatted with Decider about his three-season Slow Horses run, that action-packed Season 4 finale, his love for Gary Oldman and Aimee-Ffion Edwards, and much more. (When you’re done reading, be sure to check out Decider’s interview with Will Smith for more Slow Horses insights and Season 5 teases.)

DECIDER: Before we dive into Season 4, when you joined back in Season 2, how familiar were you with the show and the books? Were you aware this day was coming for Marcus?

KADIFF KIRWAN: I know how many seasons I signed up for, but I didn’t know the tone of the show. I didn’t know the show, the books, I didn’t know anything. When I got cast, I ordered the books and I started ferociously reading to see where we were going. And because I knew there was a certain time limit on Marcus, I was trying to stitch certain things from the beginning. From the moment he appears on screen, he’s got this chill attitude towards his addiction. And it’s not something that’s crazy for him, you know? All I knew when I signed up for the show was that it was starring Gary Oldman, Kristin Scott Thomas, Jack Lowden, and it’s about these misfits. And I was like, “That sounds amazing. Will Smith’s writing? Cool. Oh, it’s the other Will Smith? Great.” [Laughs] It’s been one of the absolute joys of my life. I couldn’t have dreamed for a better company of people to work with. I couldn’t have asked for better dialogue to say. And I couldn’t have asked for a better response from the audience. It’s been truly amazing.

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I had the pleasure of chatting with the other Will Smith yesterday. He had lovely things to say about you. He also said that you haven’t told your friends and family about Marcus dying, and you’ve even gone so far as to say you’re filming Season 5, which is iconic. [Laughs]

I’ve been lying for 13 months to everyone I know.

Who do you think is going to take the big reveal the hardest?

I think probably my siblings. My two older brothers and my younger sister are completely obsessed with the books and are completely obsessed with the show. But I was like, “We’re veering away from the books, blah, blah, blah.” For the last year, I’ve been telling them I’ve been filming the show. I’ve actually been filming other stuff, but they have no idea. So when the final episode goes out, I’m actually going to go and visit them and we’re going to watch it together. I’m going to try and sneakily record them watching it, because they think it’s Louisa who’s going to die. And I can’t wait for them to see that. It’s the world’s longest prank.

Everyone is going to miss you and Marcus terribly, but it was a very impactful way to go. He protected his pals until the very end. What was it like filming those final shootout scenes and wrapping the show?

The shootout scene itself was amazing to do — just the scale of it and becoming an action man for those two weeks as we shot. We destroyed Slough House. That was amazing. But the final, I’d say the final five minutes that we see Marcus on screen, I found it so difficult. Aimee, who plays Shirley, and I, the day that we shot the last three scenes of us together, when they’re very eerily trying to get up to Lamb’s office, and the moment where he goes decides to go back out to protect the Horses — we could not look at each other. I cried. She cried. And then when I had all of my prosthetics on after Marcus passed, we couldn’t look at each other. It was so emotional, because I thank the show for my off screen friendship with her. It’s something that I cherish amongst all the prized possessions in my life. And it felt apt that Marcus was going to go out there to try and protect Shirley. But it was really difficult to shoot because we are all so invested in the show and in our characters in the best way possible. So as sad as it was, I’m happy that it happened the way it did. If you’re going to die, you want to go down in a blaze of glory. So I’m happy it happened that way. Am I happy he’s dead? No. For goodness sakes. Mick Herron. Will Smith. REWRITE! But I’m grateful that he got that moment in the sun to try and save his pals.

So am I. So it’s safe to assume you’ll be tuning in next season to see the aftermath — specifically how Marcus’ death impacts Shirley?

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100%. Aimee and I, we text each other every day. She’s like, “I miss you. What would Marcus say about this?” I’m now looking forward to becoming a fan of the show outside of being in it. I see everyone all the time. I saw Chris Chung this morning, who plays Roddy Ho. I spoke to Roz, who plays Louisa, yesterday. Gary, we were all together at the Emmys two weeks ago, so we got to really enjoy ourselves. And I think we’ll continue to be in each other’s lives for a very long time. So I’m going to be watching them with an eagle eye.

I love that. In Season 2, you stepped in for Min and got to work with Louisa, and then Season 3 is when we saw you and Shirley team up. What was it like getting to work so closely with those two amazing talents?

Both Amy and Rosalind, they’re just incredible. They give so much of themselves with their characters. They challenge you in the best way possible. They demand a certain level of excellence, so you have to rise to that occasion. I find them to be two of the most dynamic actors I’ve ever worked with alongside Gary and the entire company. But with Aimee especially, she is a ball of fire. And with Shirley, with each line, you get singed. And in turn, Marcus is water and he’s trying to put water on the fire. So it’s been pretty amazing working with those two. Some of my most cherished memories of being on set are with them.

Looking back on your Slow Horses experience is there a favorite Marcus scene, or storyline, or day of work that stands out to you?

It was our first day. So when Aimee and I joined the cast in Season 2, our first day on set was those typical Slough House scenes with everyone. And Aimee and I were looking at each other like the new kids who joined school halfway through the term. Everyone else has got friendships, but Gary and Jack and Chris and everyone were so warm and welcoming, especially Gary. I was like, “I’m doing a scene with Gary Oldman. I’ve got like three lines, but I’m doing a scene with Gary Oldman.” And he’d come over and have a talk with you and they’d shout “ACTION!” this ferocious Lamb character would come to life. And I was like, “WHAT? I’m getting paid for this. This is absolutely crazy.” It was just amazing. They say never meet your heroes, but when it comes to Gary Oldman, if you can meet him, try to do it, because he’s an absolute diamond.

I did love seeing you and Gary at the Emmy’s in your polka dots. And in terms of Marcus and Lamb’s relationship, we saw Marcus struggling with his gambling addiction throughout the series and being very stressed about providing for his family. So watching Lamb argue for that 10-year salary for Marcus’ family in the finale really got me.

Yeah, same with me. I can’t remember whose suggestion that was. But when I read that in the script, it was so tough. When we got the scripts delivered for that episode and I knew Marcus was going, I read up to his death and then I physically couldn’t read anymore. I didn’t want to know what was going to happen after that. I really didn’t. And then my final day when I was shooting that sequence on the way to work, I decided to read the rest of the script and that probably is what sent me over the edge. That way in which Lamb is so unpredictable, but when it comes to his Horses, nothing is too much for him. So him fighting for Marcus, and for his family and his pay so they could be taken care of was so beautiful. And a beautiful sign-off from a man who saw Marcus. He really saw him. Which I think is so beautiful.

And Lamb showed his love in small ways, like those little nicknames he gave Marcus and Shirley — Little and Large, Scratch and Sniff. I’m obsessed with those. Scratch and Sniff had no right to be that funny.

So scratch and sniff was Gary. Gary came up with that. [Laughs] We were shooting the chase sequence of Season 3, and Aimee and I were running, and running, and running, and at the end of the chase sequence we run into Lamb. Then off camera he called us Scratch and Sniff and we cracked up laughing. And he was like “Well, I’m going to keep that.” And that’s where it came from.

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I love that so much. I wanted to ask about another standout episode for Marcus. He really showed his stuff in the Season 2 finale when he helped save Louisa, but tag-teaming that shootout with Shirley and fighting Duffy (Chris Reilly ) in Season 3 finale was epic.

That episode, I know it’s only like one hour on screen, but it was weeks and weeks and weeks of shoots, so it was actually quite demanding for everyone. It was a very rough autumn and winter of shooting all of that. But seeing the final product on screen was amazing, because we just really wanted to show that the Horses might be down and out, but they’re all highly skilled in their jobs. They’re all spies for a reason. Getting to do the stunt training and the gun training alongside all of that, and all the explosions going off, it was amazing. And getting to have a rough and tumble with the old Duffy was another memory of mine that I’m going to cherish on the show. It was pretty cool.

What’s something you’ve learned from your time on Slow Horses that you’ll take with you to other projects?

Just finding the human nature of dialogue of comedy and drama. Slow Horses gets it so right. It’s just finding the ease with the language in the way that we’re having a conversation now. Finding the ease with everything, just sort of rolling your way through it, taking it chilled. I’ve also just learned to trust your instincts, because with Marcus, he could have been played a very different way. Sort of big and blah, blah, blah. And I was like, “No. I think he’s a much more relaxed, softer, kinda chilled person until he needs to become a bulldog.” So just trust my instincts.

Before I let you go, is there anything else you want to say to fans who will undoubtedly be mourning Marcus after the episode airs?

Keep giving the love to the show that you have been, and get all your friends onto it. Apple TV+, you know where to find it. Just keep watching and keep rewatching, because the show is so layered and you think you know what’s going on the first time around. But when you rewatch, you realize there’s all these other things at play that will come up in future seasons. So I would say go back. Enjoy.

You mentioned earlier that you were filming other projects when your family thought you were shooting Slow Horses Season 5, so what should fans who want to see more of you on-screen be on the lookout for?

I’m in the new season of Everyone Else Burns , which is out on The CW and Channel 4 next month. And I’ve got a great new show for a streamer that comes out next year, and I’m also writing my own show with the BBC currently.

Slow Horses Season 4 is now streaming on Apple TV+. Be sure to check out Decider’s interview with showrunner Will Smith for more Slow Horses insights and Season 5 teases.

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