Jane Brennan runs her fingers through her newly spiked and darkened hair and laughs: "I'm having a ball." After a year of major roles at the Abbey, the 37-year-old actress is romping towards Christmas as Julia Melville in Sheridan's comedy of manners, The Rivals, which opens tonight. A small part in the play's subplot is just what she wants, having recently returned from playing the title role in Tom Kilroy's The Secret Fall of Constance Wilde in Melbourne.

"It's a relief not to be carrying the whole burden of a play on my back. This is a small holiday really - it's a luxury to go out and have fun."

The director, Brian Brady, is taking a slightly anachronistic approach to The Rivals. "The visual style will be quirky and eccentric, but the text will not be tampered with," Brennan says. "It's not trying to be clever, just giving it a little twist, to enhance what the play is doing. We're playing with the style, which is what Sheridan himself was doing with the stock theatrical styles of his day." Brennan would happily chat about Sheridan and 18th-century burlesque all evening, and it's clear that despite the modest scale of her part in this production, she has done her homework. Research is an important part of her preparation for her roles, to which she takes an analytical approach. "I have a very logical mind," she says, "and I want to keep learning." Over the past year she has been consistently under the spotlight at the Abbey - as the angry, disaffected Vera in Tom Murphy's The Wake, Joan in Shaw's St Joan and the stoical Constance in The Secret Fall Of Constance Wilde - opportunities that she has seized eagerly. "I may never get these chances again: I'm conscious of my actor's biological clock. The past year has taught me an awful lot. You can talk and theorise about acting forever, but you only learn by doing it.'

Isn't it a dauntingly public way to learn? "Yes, you know yourself when something is not working, and that's the worst feeling in the world. You feel horribly exposed and humiliated. Over the years I have had a few painful experiences, let's say, and people have long memories . . ."

And so does she. In fact, her own standards for her work are probably more exacting than anyone else's and self-criticism never seems far from the surface. Yet she is doing exactly what she wants to do. The daughter of two actors, Denis Brennan and Daphne Carroll, and the sister of Catherine, Barbara and Stephen, the theatre is her whole world. What drives her as an actor is the urge to extend beyond her own experience and personality. "I'm straining after some kind of creativity. I'm always trying to find ways of being more creative so that I can communicate what I do better. As an actor, it can't be just about you; you have to exceed yourself. "But," she shakes her head with frustration, "I still can't quite manage to exceed myself. "With every new role you face the challenge of making it your own. With a part like Shaw's St Joan, for example, you face your limitations, and that is a very humbling experience."

With St Joan she also felt the weight of theatrical tradition. It's a role that many leading actresses have measured themselves against, and which bears, in this country, the imprint of Siobhan McKenna, "People came to it with all sorts of preconceived ideas about Siobhan McKenna, but I knew I couldn't play it like that. This was not going to be a great rhetorical performance. At first I thought, how am I going to play this young, pious girl? I read the transcript of the trial and that was my guide. I wanted to play her as a real person, full of common sense. Joan came across as a very uncomfortable person to be around. She spoke her own mind."

IT IS never easy to talk about the process of acting, particularly about that mysterious alchemy that occurs when an actor is not merely impersonating a role, but inhabiting it. University library shelves groan with theoretical volumes about the actor's art of interpretation and creation but, as audience members watching a performance, we can often feel unclear about where the director's contribution ends and the actor's begins. "Nobody understands actors," Brennan says firmly - "except other actors. I'm often surprised by the lack of knowledge among critics about the process of acting. It seems to me that they often don't know how to write about acting, about the choices an actor makes about how to play the role. Actors have a huge input into this. Most good directors let the actors make their own choices about the role and then steer them."

The casting decisions are the director's, of course, and Brennan often finds herself taking on roles that require considerable emotional restraint and control. The part of Constance Wilde, the stoical wife of Oscar in Tom Kilroy's play, is very demanding for that reason: "There's no release. The feelings are completely pent up, and I find that they stay with me for a long time afterwards." Similarly, the repressed, crossdressing waiter, Albert Nobbs, which she played in Simone Benmussa's production of George Moore's The Singular Life Of Albert Nobbs for Druid two years ago, required enormous self-control. Presumably it is her qualities of cool rationality and detachment that prompts directors to cast her in these roles? "I do internalise things, certainly. I am not presentational," she says. "The most frequent criticism I get of my acting is just what you have mentioned - a lack of emotion."

As Vera, the hard-drinking, passionate, angry returned emigree at the heart of Tom Murphy's play, The Wake, which opened last January and is travelling to the Edinburgh festival next year, she had a chance to burst out of the straitjacket. "This is a much more complex and draining role than Joan, in fact," she says. "I didn't really know how to prepare for it."

The fact that the playwright is her partner must have helped a bit? "Yes, I was familiar with the territory, broadly, and I have a feel for Tom's style of theatre."

This style, in which everything is writ large, always carries with it the danger of melodrama. "Actors have to be very careful not to topple over into caricature. Playing in a Murphy play is not a million miles from playing 18th-century comedy. The characterisation is big - sometimes larger than life - and you have to work from the outside in. You must find the right performance style, and then move inwards to discover the truth that supports it."

The part of Vera was not written for her, she says, and she was cast by Patrick Mason, rather than Murphy. "I see a lot of Tom himself in Vera - the feelings about home, and family, the humour and the anger. I approached it with a certain amount of trepidation," she says, laughing. "But in rehearsal, Tom was always available to everyone. There was no question of my getting special attention from him. We agreed not to discuss it at home at all. But he comes to everything I perform in and gives me his comments. If anything, I would say that he is my biggest critic. " After herself, that is.

The Rivals opens tonight at The Abbey.