What was it about this project that made you jump at it?
Richard [Eyre] really. We've known each other forever, but we hadn't worked together until Ghosts [in 2013], and something is just good about us in a rehearsal room. So when he said I want to do Long Day's Journey… it was just a no-brainer, everything about this job is the reason I do the job.

Why is it that you and Richard work?
It's very hard to pin down. He casts brilliantly, that was true of Ghosts and it's true of this. The other day he said something wonderful, we did an act and he said: I feel like I don't have to direct, I feel like I'm just conducting a finely tuned string quartet. With Richard it becomes a collaboration and I suppose I bring a lot to the table. I am very free in rehearsals and not frightened of the emotions, so I think he likes that.

Will you do a third?
I don't want it to stop! We talk about things all the time, hopefully there will be a third and a fourth and a fifth.

It's a good year to open in Bristol…
Yes! It's the Bristol Old Vic's 250th anniversary and I like the fact that we aren't so London-centric. The stage at the theatre is stunningly beautiful, I've not been there before now, and it's quite small and feels like its wrapping around you, so it'll be perfect. As if the audience are in the living room.

Ghosts was emotionally exhausting, Long Day's Journey is also very demanding, how are you finding it?
It's the hardest play I've ever done. Ghosts was an hour and a half and we did it straight through without an interval. Once I came on stage, I didn't leave and it gathered an emotional momentum – my character didn't start the play unhappy. But the scale of this is so much… we're not cutting it, it is truly epic and it's difficult for my character all the way through.

How have you coped with the lines? Mary Tyrone has a lot to say…
I decided I would do my very best to learn it before we started because you just can't learn this much during a five week rehearsal. She's one of those characters who almost has to do circular breathing. She just keeps on, because she's on morphine. Eugene O'Neill's play has come from a very personal place, it's his own family onstage and he certainly writes a woman who is liberated [by the morphine] to speak, and boy does she. I am not really pausing for breath.

Is Mary the mother of the family?
Yes. At the beginning of the play you realise she's been in a sanatorium for her addiction which was triggered after she lost a child. She was prescribed a painkiller and it's 23-24 years down the line and she's still battling. It's about the breakdown of that family. The proverbial shit is hitting the fan.

I've hardly done any Shakespeare, I would like to do more

What has it been like working with Jeremy Irons?
He's absolutely lovely to work with and I think he's just perfect as this matinee idol James Tyrone. I think he's loving the very different muscle that you have to flex from being on set where you get through five lines and suddenly someone shouts cut.

You have also had a very strong working relationship with Mike Leigh
Yes, he has been instrumental in my career. I think he's partly due to why I'm still going. Because nobody can say – oh she just plays posh or working class or she plays rough or smooth or glamorous. Mike allowed me to jump all over the place.

Is there anything you'd like to do more of?
I've hardly done any Shakespeare, I would like to do more. I've missed a lot of his characters, but there are still plenty out there. But doing this American play now, it's just phenomenally exciting. The language is just utterly thrilling and the audience should not quite want to watch.

Ghosts was a huge success, was it hard to follow something like that?
I don't think of it like that. When I said I was going to do Long Day's Journey a couple of people did say: of course that's the obvious part after Ghosts. They pointed out that it's the next level of pain, grief and misery. Of course the play I had done before Ghosts was Grief, the Mike Leigh play at the National, and the clue is in the title there.

Do you take it home with you?
No, I honestly don't. By the time my foot had hit the first stair at the Almeida and at the Trafalgar after Ghosts, it had gone. I can be tired, but I don't get struck by the blues.

Are you happy mixing up film, TV and theatre?
Yes, I still always judge it by how good the piece is. I am in a very strong position in theatre, I feel like it's as good as it gets, I am playing big leading roles with the best directors that we have. I choose television very carefully. I recently did River because Abi Morgan wrote it, then Mum – which isn't out yet – is really lovely. It's produced by the same people who did Rev and written by Stefan Golaszewski.

Have you ever regretted saying yes to a part?
Oh yes. I won't name it, but I once wanted to invest time in promoting a film, but I knew that I had to earn some bucks before I went. So I did something that I might not have done. But it earned enough money to keep me going for a few months. And that situation might happen again, you never know. We were saying in the rehearsal room - none of us have pensions!

Long Day's Journey into Night runs at Bristol Old Vic until 23 April.