Why haven't you done Shakespeare before?

It's not front and center in Ireland the way it would be in the English psyche. I did come to London to study acting and we did deal with a lot of classic Shakespeare, but it wasn’t at the top of my list. A lot of my influences came from ’70s American films.

The director, Justin Kurzel, suggested that Macbeth had post-traumatic stress disorder, didn’t he?

As soon as he said it to me, the penny dropped, a heavy penny. It resonated so much in the character and it was a massive key for me to get into his head. There’s evidence in the text. Lady Macbeth is like: Don’t be alarmed, we’ve seen him have these fits before. So the idea of seeing the witches, were they part of hallucinations? The fact that he can’t get any sleep. These are classic symptoms.

What did that approach give you to work with?

It was huge that this character was already mentally fragmented [from earlier battles] when we meet him. There’s a familiarity with blood and killing. As soon as he kills Duncan, there’s a clarity for him because it is something so familiar, like it would be for an alcoholic who sits down and has that first drink after being on the wagon.

The films starts with Macbeth and his wife burying their infant child, although the standard reading doubts that they had children.

Historians say they don't know and many allude to the fact that maybe Macbeth can’t even sire an heir, but how do you play that? You can definitely play a couple who have lost a child and have drifted apart because they haven’t been allowed to mourn. He’s been away campaigning for months on end. We treated that breakdown of the relationship as a fuel to doing something so heinous, which is the murder of Duncan, which would hopefully bind them together again.

This version also resists the traditional idea that Lady Macbeth manipulates her husband into doing terrible things out of ambition.

Yeah, that's the bad old woman again, isn't it? None of us found that as interesting as the fact that she is actually the most sacrificial character, because she asks that all her maternal instincts be stripped from her so she can do this for her husband.

Your performance is restrained. The camera is close and your voice is low. It's not the typical raging, yelling portrayal.

Justin and I really wanted to stay away from that. The idea was, this is a man talking to demons or talking to various things that don’t exist. What’s his internal motor like? We were given the great benefit of film. There’s a much more intimate feel of the madness. The language in Shakespeare is so dramatic, to play it dramatically just becomes a mush of madness.

You filmed this before "Steve Jobs." Was there anything from playing Macbeth that you carried over to Steve Jobs' character?

I didn't, to be honest. Once I finish one character I kind of kick it to the wayside and then really immerse myself in the next one. What I did see as an immediate parallel was the writing. Aaron Sorkin writes to a specific rhythm and cadence, and if you obey it, that unlocks a lot of the character’s emotional state.

Is there a ruthlessness and ambition, like Macbeth's, in Steve Jobs?

I never saw Steve Jobs as ruthless. I saw him as uncompromising and extremely driven. I don’t think he was driven by money, but because he wanted to change the world—and he did. That people take their iPhones into bed with them, and it’s the last thing people do before they go to sleep at night and the first thing they do when they wake up—that's pretty extraordinary if you think about it.