Warning: This article contains major spoilers for the Season 4 finale of Slow Horses

The actor, whose character dies in the Season 4 finale of “Slow Horses,” had to keep the secret a secret for more than a year, even from his own family. “You’re the first person I’ve told about this,” he said in an emotional Zoom interview.

If you've already watched the finale, now streaming on Apple TV+, you know that it's Marcus Longridge (Kadiff Kirwan) who dies in a shootout with an assassin at Slough House.

TheWrap spoke with Kirwan about having to lie to his family, friends, co-stars in the London stage production of “The Hot Wing King” and the media for so long. During the interview, he praised the series' lead actor, Gary Oldman, as “the kindest, most generous, funniest guy” and Oldman's gruff character, Jackson Lamb, as “the ultimate papa bear.”

TheWrap: When did you find out Marcus was going to die?

Kadiff Kirwan: I knew it from the beginning. I didn't know how he would die, but when I signed up for the show I knew I would write three books (by Mick Herron). I tried to do it harder and try to stop them from killing me, but I'm so, so grateful. Oh man, what a death. If you go, you want to go the same way, right?

How did you feel about it happening off-screen?

I think that's the way they did it with the horses. There is a kind of respect and awe for them that I am so grateful for. The cut we made of it is really something special. We did a lot of stuff around the death, and I think the way they did it is incredibly tasteful, and it's not just about him dying, but how that affects everyone others in the building and everyone else, especially Shirley (Aimee-Ffion Edwards) and how Lamb still has Marcus' back in the end to make sure his family is taken care of.

Do you feel a bit suffocated here when you think about it?

Literally, it's the first time I've said this. I didn't tell my family that my character was dying. Sharon, you are literally the first person I've told about this. So when the final episode comes out, I'll actually go and watch it with my family. I'm going to record it as I watch it because I haven't told a soul since.

Wow. Marcus is killed in the books too?

Yes, that happens in the books. But the nice thing is that the way they source material is fantastic, but they've managed to twist and turn things in certain ways. But it made sense that Marcus would care about the development of the show, but also the development of the other people.

It's so devastating for Shirley. They had a sort of antagonistic relationship where they insulted and belittled each other, but they were truly devoted to each other.

Complete. And it's so rare to experience a workplace relationship that is completely platonic and one of mutual respect and reverence. I love their relationship. To be honest, my scenes with Aimee-Ffion Edwards as Shirley remain some of my fondest memories on set ever.

Last season you both gave it your all in the finale and killed all the dogs, you seemed invincible. And then this one guy (takes you out)…

This French assassin comes and he's crazy because the cliffhanger is obviously the way you hear the gunshot. You don't know who's gone. And honestly, I'm so disappointed.

At the beginning of the season we had a fake out suggesting that River (Jack Lowden) had died. So do you think that makes people think, “Oh no, Marcus can’t really be dead?”

There's no way people think (Marcus would die). My family and I made a parody video about who will die. We are all in this video, my siblings, my family. They’re all like, “Oh, it’s going to be Louisa (Rosalind Eleazar). Min's gone, so something's going to happen to her.” And I thought, “Oh God, imagine if it was Louisa,” and I just play along, knowing I'm the one with my mind all the way up on the side of the head office of Slough House.

You have a great poker face.

Yes. Plus, I'm under contract with Apple and didn't want to upset them. They have been very, very good to me and I hope that they will continue to be good to me in the future. For me it was a test to keep the secret. So I'm really glad I was able to keep it for over a year.

That's a long time.

I did all the press. Season 5 is already being planned, of course, and I'm not in Season 5. But even the play I did at the National Theater just two weeks ago, throughout the whole play (the cast was like) “Oh, we just saw “Slow Horses” is being filmed.” I thought, “Yeah, I'm filming all day and I have a show tonight.” I don't know where I get my energy from. “I lied to everyone but you.

I feel very honored. So it's like being a spy.

Right. They keep the secrets. It's beautiful. And I love that there are very few television shows that have this tension and this appetite. I'm just so happy to be a part of it.

Looking back, what was Marcus' favorite moment in the series?

I probably walked through the damn window of that store. (Laughs) I would say his last scene, the shootout and his last scene. Because, and this is no offense to anyone, if he had had the conversation about dating someone like River or Ho (Christopher Chung), he might not have dated. But because it was Shirley, he went there because there's no way he's going to let anything happen to her or the other horses. So he died for the right reasons with the right intention.

Who do you think told Marcus' wife that he was dead?

Yes, I wonder that. Personally, I think it would be Standish (Saskia Reeves). I think Standish is probably the most emotional of all the horses. She is best equipped to deliver bad news. I think it would be too much for Shirley. And I think Lamb did it in his own way by making sure that Marcus' family received the financial support of the service because he was killed in action, in the line of duty, right? Oh my god, look at that. I just received a notification from Aimee-Ffion Edwards on my phone and watch. She can't stay away. (Laughs)

Everyone else also had to keep this secret to themselves.

Every single person. Oh my God, on the red carpet at the Emmys, Gary and I were doing an on-camera interview and he almost slipped up and I saved it. It was so nice because we both looked at each other and couldn't laugh, but we just wanted to laugh because after all, we were there to promote Season 3 for the awards they were nominated for. Season 4 is currently out and season 5 is in the can. So it was like, “What season are we talking about?” Luckily we didn't get that far, but it was very, very funny and we shared the look that will stay with me forever.

Jackson Lamb pretends to be very callous. And then we see that he actually cares in his own way.

He cares 100%. He's like the ultimate papa bear. He really is. He is the great protector of the secrets of the MI5 service and the staff of Slough House. I think a better fallible character has never been written. And the playfulness that Gary brings to it is unmatched.

It was one of the joys of my life to not only share the camera and stage with him, but also to develop a friendship with this man. I would honestly do anything for Gary. I think he's just the nicest, most generous, funniest guy, and for someone to just… you know, because when Aimee and I started, it was Season 2. It's like starting school in the middle of the semester in a new school start where everyone is already in their groups, and he just greeted us with these anecdotes and these stories, and his tongue firmly in his cheek. They don't make them like that. They really don't.

It's such perfect casting.

That's it really. We laugh a lot on set. I pray that Apple releases the bloopers at some point, because they haven't yet. And I think if we put in a petition, they might do that, because there's some funny, funny, very funny stuff that we've done on camera that's been caught.

I was sorry to see you go. I hoped until the last minute that Marcus was somehow alive, just like River.

(Laughs) I'm still trying to convince them that I'm coming back as a ghost or as Marcus' twin brother. It could be the first for Apple. Come on, you're so inventive. Just let it happen.

All four seasons of Slow Horses are streaming on Apple TV+.