Warning: This post contains spoilers from the season finale of The Undoing.

The Undoing revealed its killer in its season finale. After six episodes of twists and turns, we wound up back at the show's first suspect: Jonathan Fraser (Hugh Grant) killed Elena Alves (Matilda De Angelis).

After Grace (Nicole Kidman) took him down on the stand, Jonathan — realizing he was going to be convicted — took his son, Henry (Noah Jupe), for one last drive to make sure he understood his legacy. Then, he was planning to jump off a bridge. But when Grace arrived, Jonathan didn't go through with it, and he ended the series in handcuffs.

EW spoke with the guilty party himself, Hugh Grant, about playing Jonathan.

ENTERTAINMENT WEEKLY: At what point did you know he was guilty? Was that part of the initial pitch?

HUGH GRANT: I was sent episode 1, which is all they had written at that stage, and of course my first question was, "Well, did he do it?" And the answer was yes. And that's what made me want to do it because it's an infinitely juicier prospect for an actor to be a narcissistic sociopath than just the good doctor husband who shagged the wrong woman and spends six episodes apologizing.

That was one of my questions: Is he a sociopath? Does he feel empathy? I imagine you had to decide for yourself exactly what you were playing.

Yes, because I needed to be as convincing as I possibly could when I'm expressing love for my wife and my son, for instance. The true psychopath can't do that, they don't feel any love for anyone else, especially the narcissistic ones. I needed to be convincing for the sake of the story. If he's not convincing, everyone's going to say, "Well, it's him," because the circumstantial evidence was heavily stacked against him anyway. I had to be as convincing as possible, so all that told me was that I had to go right to the most extreme end of these types of characters where they are. And I did the research, they can be incredibly plausible, to the point where they actually believe their own lies. So he believes that he didn't do it, even though he knows intellectually that he did. In terms of his feelings for his wife and his child, I think that's all part of the narcissism. He loves them loving him, that's like rocket fuel to him. He must have it. That's why he's so desperate in the car at the end to make sure his boy is going to love him even after I'm found guilty and sent to prison or commit suicide or whatever I'm intending to do there. He's also such a narcissist that he can't accept the fact that he might be a psychopath. He must be a wonderful loving father and husband and so he acts it in a desperate bid to convince himself that he's this person. But deep down he knows who he is — he's a completely cold-hearted, narcissistic, violent, terrifying human being.

But then he doesn't jump when Grace shows up, which I thought was interesting, because arguably, the selfish thing to do would be to jump. But he stepped down.

Yes, I think because he would not like to die with his wife thinking he was a selfish bastard who killed himself in front of his son. He still wants their love and approval, because this is part of his narcissism and it's so extreme by that point, that when he gets down off the rail and walks toward Grace, he's actually smiling because he thinks yeah, maybe I'm going to get a hug here and I can still charm this woman back into my orbit. And, of course, she recoils and takes the boy away. I think that's the first time, really, in Jonathan's life that he's completely failed and rejected.

I loved him telling his son that it wasn't the "real me" who killed Elena. It was a look into how he's been compartmentalizing everything all season.

Yes, exactly. He can no longer pretend it didn't happen. He says, "Sometimes we lose ourselves" and he's got almost a medical justification for it, like it's a dropout of personality and just because we've done that we must not think our normal personality doesn't exist. But it's all bulls---.

Nicole has talked about the intensity of director Susanne Bier's directing style. What was the actual filming of those moments like for you?

Susanne didn't shove the lens right up my nostrils and in my eyeball like she did with Grace because it's Grace's story, really. So I wasn't intimidated by her camera work. I was a little intimidated by the whole finale because it had been hotly debated and rewritten and reconceived and the question was: Would it work? Particularly when the stage direction is "he's losing it." That's extremely dangerous. You can be ridiculous in a millisecond. Then I proposed this idea that they sing this song — which is actually a song my cousins and I used to sing as kids on holiday at the seaside in England — and having shot it, I've spent the last year and a half thinking, "Oh, we shouldn't have done that, that's going to be ridiculous." But actually, I think it does work, especially because Susanne has very cleverly put it in counterpoint to the actual killing. The cutting between that silly song and me smashing poor Elena's head is pretty good.

So when he takes Henry on that drive, is any part of that out of love for his son?

I went through phases of thinking maybe Jonathan's narcissistic sociopathy is as a result of damage and if it's a result of damage then pre-damage, he might've been capable of human emotions and there might be a flicker of that real stuff for his son. But if I'm really honest, it's all about Jonathan and his apparent love of his son is really just a love of his son's love for him and addiction to it.

I'm so fascinated by the idea that you could know someone for so long, and not know they're a psychopath until something like an affair triggers it.

Yeah, it's terrifying. I think that's partly what made the series quite compelling to people is that lurking dread of, "My God, do I really know anyone? Do I know my wife? Do I know my father? Do I know my son? Could they secretly be absolutely diabolical deep down?" What would happen if we looked at everyone's internet history, what terrible dark secrets would we find in people we love? I think that's part of the lure of entertaining films and television.

So much of the series' big twist was on your shoulders. Did you feel a pressure to make the audience believe Jonathan wasn't guilty?

I didn't have to make people think he's innocent, I just had to make them think, like Grace, that they're open to the possibility that he's innocent. And in that, I realized, reading Twitter over the last six weeks, I was helped by the fact that audiences are so canny and experienced now that they all took the view: Well it can't be the most obvious suspect because that's too obvious, it's boring. It must be something much cleverer than that. Which then made me dread the screening of last night's episode because I thought they'll be so disappointed because it's just the man hiding in plain sight, but on the whole I think they liked it. 

Sometimes the obvious choice is the biggest twist.

It's also the end of the true story of this film, which is, "Do we really know the people we think we know?" Grace the psychiatrist at the beginning of the show is lecturing her patients about, "Well, you kind of knew your husband wasn't as nice as you thought he was, but you blinded yourself to it." Of course, she's done exactly that to herself. So this is the correct ending to that story, as it was the ending to the novel. I think the whodunnit element was all a lot of fun and juicy and brilliantly conceived by David Kelley, but it's not ultimately the point of the film.

Many people are calling this your greatest performance. Did it feel special making it?

I knew it was very high pedigree and I was privileged to be a part of it. You can't get a better television writer than David Kelley, Susanna Bier is an aristocrat of cinema, and Nicole Kidman, who comes dripping with Oscars, and Donald Sutherland, everyone. It was classy. It was a strange part in that really, I only got to play the true Jonathan for one scene, which is when I go around and have sex with Elena and then kill her. That's the real unmasked Jonathan. The rest of the time it's all about masks and how many masks he's wearing.

That's true. So many actors talk about the difference between television and film is that in television, you get to live with a character for longer. But you really just got to live with a character's lies for longer.

Yeah, exactly. Levels and levels of lies. I always have thousands of notes all over my script. They're explaining to me what the character's thinking and their motives for everything they do and say. With this character, I had to have a double set because there were two people having thoughts. One was real Jonathan, who I called John Boy, J.B., and he knew he was guilty and he knows he's a monster. And then there was I.J., Innocent Jonathan, who is just interested in deceiving people and actually believes his own lies. And they overlap, but they're not the same person and so it was quite a difficult mind f--- actually, trying to navigate my way through how I'm feeling in any given moment. But always there was this imperative that I couldn't give the game away to the TV audience.

Related content:

• Why a second season of The Undoing is unlikely
• The Undoing finale recap: She f---ed us
• The Undoing killer revealed — see the best reactions

This story originally appeared on: EW - Author:Samantha Highfill