Niamh Cusack: The Rehearsal has given me a taste for playing bitches

Niamh Cusack, a member of the Irish acting dynasty whose credits include The Curious Incident of the Dog in the Night-time and Juno and the Paycock, has returned to Chichester Festival Theatre to play The Countess in Jeremy Sams's revival of Anouilh's savagely funny 1950 comedy

How's the production going down so far?
It's going down really well. I think people are quite thrilled by it. It's a brilliant play and Jeremy Sams has done a fantastic version of it – at the beginning you think it's something akin to Coward, and then you realise it's more like Rattigan. There's real heart to it. It's really about a marriage and indeed a society that has cracks in it, and they begin to show as the play progresses.

The play straddles two eras – could you explain more?
The characters in the play are rehearsing an 18th century play, but the rest of the time it's set in 1950, centring on this rather exclusive group of people who are disrupted by a very truthful young girl. My character's husband falls in love with her, and she cracks open the fault lines in the marriage, which is in itself a metaphor for the society of the time.

Tell us more about your character, The Countess
She's a woman of a certain age, married to a younger man, and this young girl is an opponent she never thought she'd have trouble with. She's somebody who plays all the society games and organises great parties, but in the face of truth she finds herself playing pretty dirtily – she's down in the mud by the end of it. It's a real fall from grace.

What's your route into the character?
You find things in your character that link to things in you. This woman is older, she's in a place in her life where things are changing and she's trying to get to grips with things. I'm a woman in my 50s, so I can relate to her. But the extremes she goes to in order to survive I wouldn't go to!

Are you glad to be working in Chichester again?
I love Chichester, both as a performer and audience member. And we've got an incredibly warm-hearted and lively company on this production, they're really inspirational to work with. The age range starts at 25 and goes up to 60, so we're a broad mix which I think is always very enriching.

I've read that you didn't always want to be an actor?
I wasn't encouraged to do any acting at home, and didn't do any at school. This was partly because in those days drama wasn't a very big subject in Ireland, which is strange when you think of all the big Irish dramatists. Anyway, I was good at music and I won a scholarship to the Royal Academy in London [as a flautist]. My parents were thrilled that one of us wasn't going to be an actor!

So what happened?
After I studied I couldn't get any work. So I went to the City Lit with the intention of doing a French course, but it was full up, and they had spaces on the acting course. I'd just been to see my sister Sorcha in Three Sisters and I remember thinking that the character of Irina was one I really knew how to play. I think the seed was sown then, and I did the course and absolutely loved it. After that I did a year at Guildhall and then started working at The Gate in Dublin.

Do you keep up the music?
I still play the piano, but not really the flute anymore. When you stop practicing flute it really shows, and it's quite hard to hear yourself play badly. But I really enjoy playing piano duets with my sons.

Is acting comparable to musicianship, or do you find it more instinctive?
I do find it more instinctive, but then I think for a real musician music is instinctive. But you always have to practice both, which is why it's hard for actors when they don't have work. At least a musician can practice at home.

Any dream roles, or roles you wish you'd played?
I never got to play Viola [in Twelfth Night], or Sonia in Uncle Vanya. Though I did play Sonia in Brian Friel's Afterplay, which was a delight. As for roles I'd like to play, maybe Lady M, or Cleopatra. Being in The Rehearsal has really given me a taste for playing bitches!

The Rehearsal continues in the Chichester Minerva Theatre until 13 June