Actress Susan FitzGerald loves to get the audience laughing. But good drama can make political points at the same time, she tells Christine Madden

The interview has only just started when the telephone rings. Susan FitzGerald apologises and goes to the next room to answer it, leaving me in her airy, spacious kitchen. Candles and cake on the wooden table create a celebratory, convivial atmosphere that even the parallel presence of my notebook and recording equipment can't dispel. The scent of coffee brewing hangs in the air like a promise. The lull in the room feels like the hush before the curtain goes up on a performance.

FitzGerald ends her phone call and reappears, stage right. "Sorry about that," she apologises. "I'm going out later today. It's my birthday." Apart from her soft-spoken, genteel manner, FitzGerald is a career actor, whose life and job don't exist in exclusion; a day's work is a day's play. "I always wanted to be an actor. Never wanted to do anything else," she states. "By the time I was about 10, I just knew that was what I wanted to do."

Her Irish parents brought her from England to Ireland when she was a teenager. She went to Trinity College and subsequently graduated into a society where "everybody was unemployed". Prospects were not rosy in the 1970s, but it never deterred FitzGerald. "I am so pleased that I didn't leave Ireland when I finished university. Some of my friends did, they didn't think of staying at all, but I never did. At the time I was just newly married, and both my husband and myself decided we just wanted to work here and, whatever was going to happen, to try and improve whatever we found in the theatre."

The husband, Michael Colgan, from whom she is now separated, became director of the Gate Theatre in 1983. FitzGerald has also nurtured a long-standing relationship with that theatre, only recently looking outward to work mostly outside it for the past two years, in such productions as Edward Albee's The Goat, Tejas Verdes by Fermín Cabel, and Des Bishop and Arthur Riordan's Shooting Gallery, for which she received an Irish Times Theatre Awards nomination for Best Supporting Actress.

Next week she returns to the Gate to play Mrs Culver, the mother of the main character of The Constant Wife, by Somerset Maugham. The play copies Wilde's glib cynicism yet with none of his sentimentality. Constance, the betrayed wife, is "completely, coldly logical", as FitzGerald describes her. "She's charming, she's delightful, she harbours no rancour. She's able, with her quick thinking, to set off little explosions everywhere."

Although Maugham wrote the play in the 1920s, the production takes place in the 1930s, "because [ costume designer] Peter O'Brien thought the frocks were nicer then". But the piece oddly retains its shock factor as an examination of the convention of marriage through a utilitarian, economic logic. Misplaced emotion becomes an unaffordable luxury. The incongruity between how we behave and how we think solicits laughs but also exacts a reassessment of core values.

Mrs Culver "thinks almost as quickly" as her daughter, says FitzGerald, "but she's more constrained by appearances, and how you would deal with social mores. When someone tells her she sounds very cynical, she says, 'Oh, I hope not. But when women are alone, I don't see why we shouldn't tell the truth now and then. It makes a rest from the weary round of pretending to be something that we quite well know we're not'."

COMEDY IS a favourite medium for FitzGerald, for whom it seems to push all the right buttons. "There've been nights when I've been doing comedies, and I just thought, this is the best job on the planet. You stand on the stage, and you literally have to wait for the audience to stop laughing before you can go on. Every night we'd get to the same point, and we'd have to stop for about a minute while the audience stopped laughing."

In the midst of all the hilarity, keeping a straight face "requires so much discipline", FitzGerald acknowledges. And then, "some of the comic roles are dramatic" as well. After appearing in Alan Ayckbourn's play, Just Between Ourselves, she says, "I still have people who come up to me and say, the saddest thing they ever saw on the stage was this character I played, whose family were pushing her aside all the time. And I remember the doorman saying he would see the odd family leaving because the husband couldn't take what he was looking at, knowing that he did that to his family. Just the idea of bullying in the home - Ayckbourn captured it. So you can combine comedy with tragedy as well."

The intimacy, the immediate connection and reaction that theatre can achieve makes the sector particularly exciting for FitzGerald, who prefers it to film. "You're not alone on stage," she asserts. "You work in tandem. You feel the audience's heartbeat, and you know they're there, they're laughing, and they're shocked, and they're doing all the different things that audiences do. You are so connected."

In Tejas Verdes, FitzGerald again had the opportunity to play with audience reaction - but had the tables turned on her as well. "I played someone whom you thought was really nice, until you realised that, actually, the friend that she's talking about, she betrayed. You don't realise that until the end of the play. And then you've got to get your head around how she could have done that. And then she tells you why: they tortured her child in front of her. And the audience is taken one way, then dropped down."

But in one of the question-and-answer sessions after a performance, when a woman in the audience expressed that she felt the woman was justified in what she had done, that under the circumstances she had no other choice, "a young man stood up", recalls FitzGerald. "And he said, 'Excuse me, I disagree. My friend was in this situation in Chile, and he didn't tell. And they tortured his child in front of him, and he didn't tell because he knew his friends would be dead the next day.' There was a terrible hush in the auditorium."

Had the experience come earlier in the run, it may have affected FitzGerald's interpretation of her role.

The Goat, in which she also recently appeared, also struck a political as well as social blow. "Albee wrote that in response to the desperate anti-liberal sentiment in the press after 9/11. They literally attacked Susan Sontag in the New York Times for having defended the idea that perhaps dialogue had to be opened between East and West. They might as well have put her on a barbecue. And Albee thought, here is the liberal intelligentsia of America attacking one of their own. There is obviously a point at which everybody says enough's enough. So he chose a liberal, successful family in New York. And this woman [FitzGerald's character] has her point. I think you'd find a lot of women would agree with her. She won't go with the goat."

The production thrilled FitzGerald on another level as well. "It was like getting into a Maserati as a script. Dzhoom, it just took you off."

Slipping into the American vernacular and the crisp otherness of its culture turned into an almost sensual pleasure for her. She would welcome more experience to perform in different milieux. "I would love to tour more. I would love to use my acting to engage the rest of the world, but not move from Ireland; I don't ever want to move from Ireland. But I would love theatre to travel the world. And if you're in a festival, you see other people's work, it's cross-pollination."

TRAVELLING AND bringing her experience of theatre to other countries opens other benefits as well. "Sometimes foreign directors see you in a different light and open up areas that perhaps you're not so elastic in."

And when you work with those unfamiliar to you and your environment, it can occasion unexpected freedom. "I think sometimes the greatest challenge that an actor can face is overcoming other directors' and producers' perceptions of your limitations and styles. Actors have to be carefulthey don't restrict their parts into a certain genre, that they keep breaking boundaries and keep breaking other people's expectations."

Largely through comedy, FitzGerald has managed to do this, and keep herself in work at that awkward over-35-but-under-50 age, when so many actresses discover suitable roles thin on the ground.

"Comedy is the great ticket, it opens so many different, extra doors. I've never really been a sort of femme fatale type of actress. I did that role in Les Liaisons Dangereuses, which was quite interesting. But on the other hand, if I were an actor who had always been cast for beauty, maybe that would be a more problematic thing to get through when you're in your 40s and 50s, because they're always going to look for the New Young Thing. Doing so much comedy in my life has actually eased that transition beautifully. And been such fun to do."

Which brings her back to the Gate - "a lovely place, a lovely, happy building" - and her role in The Constant Wife, a comedy with a sting in its tail.

"Constance," FitzGerald says of her character's daughter, "is deeply shocking, but so very satisfying, even to a modern audience. It is breathtakingly refreshing what she decides to do."

You sense that, given FitzGerald's quiet energy and intelligence, whatever she undertakes, she will continue to seek out those refreshing roles that delight as well as provoke her audience.

The Constant Wife opens at the Gate Theatre, Dublin, on Tues, June 6