His CV reads like an identity crisis. So is Risteárd Cooper, of Après Matchand, now, 'Auntie & Me', a mimic, comedian or actor, asks Peter Crawley

Who does Risteárd Cooper think he is? There's something both riveting and disconcerting about watching this thoughtful man suddenly transform into somebody else. Tommie Gorman, David O'Leary and Fintan O'Toole sporadically intrude, Cooper's voice melting into devastatingly accurate impressions while his face morphs. Then, just as suddenly, the intruders disappear.

Cooper's CV reads like an identity crisis on paper: a mimic, a comedian, an actor, a comedy actor. It follows the erratic trail of a third of the transcendentally funny Après Match troupe, who increasingly finds himself cast in the leading role of straight theatre. It will be updated tomorrow, when Morris Panych's dark comedy Auntie & Me opens at the Gaiety Theatre in Dublin and the troubled character of Kemp rushes to the deathbed of his long-estranged relative. Seemingly at the end of her mortal coil, the old dear proves impertinently slow to shuffle off. "I'm concerned about your health," says Kemp. "It seems to be improving."

So do the chances for a reconciliation of the contradictory strands of Cooper's career. "I hadn't seen it like that," he says, but his bright eyes are alive to the notion. "What's brilliant about the writing is that those worlds" - comedy and drama - "seem to meet, and while there is a huge sensitivity in Kemp, he has been hurt and damaged. What he does with that is to deflect things. For there to be comedy about the world he sometimes detaches himself from reality, because it's too horrific for him, and he tries to make it funny." Cooper pauses. "Now, I'm not saying that I do that."

The skills of a mimic are rooted in detachment, and Cooper is a brilliant mimic. That ability to step back and observe a person's traits, to tease out the subtleties of a persona and then assume them, or exaggerate them, suggests a certain critical distance.

Writing about his most recent play, Hitchcock Blonde, the playwright Terry Johnson thought his own skills were closer to those of an impersonator than to an author, owing more to Rory Bremner than to Eugene O'Neill. "I imagine Bremner would find a straight role quite tough," wrote Johnson, "because acting demands not only his precision, his ear and his dexterity, but also an honest portrayal of self." Cooper is not so sure. "There is a huge amount of well-known actors who are also very good mimics, but you just don't know that about them until they emerge with an impression or two. I really don't think [the skills\] are mutually exclusive."

In fact, Cooper believes that if impersonations come easy to him, people may suspect that he finds acting a doddle. "But it's a very different kettle of fish, walking onstage to make people laugh. Somebody said to me before that it must be so scary going onstage to make people laugh. It is quite scary. But it's equally scary to go onstage and deliberately not make people laugh." Indeed, recent roles in the Gate's production of The Eccentricities Of A Nightingale and Doldrum Bay, at the Peacock, have taught him the value of a comfortable silence.

Cooper's close friend and working partner Barry Murphy, the comedian who is the Frank Stapleton to his Bill O'Herlihy in Après Match, is incisive about the differences between the two traditions. "There is a standoffishness between actors and comics," says Murphy. "Comics come from a dirtier background in many respects. There's more pubs and less cravats, tights and spear-holding. And he's in kind of a weird place, because he has a foot in both camps." And what does Murphy think about Cooper's straight roles? "I saw him in Arcadia, which was fantastic. But I've seen him in some tights, and I wouldn't go back."

Whatever about the tights, Arcadia fits snugly with Cooper's work ethic, one steeped in training - although Cooper doesn't like the word - at Trinity College in Dublin, on its acting programme, but also informed by the flexibility of the comic improviser. On the opening night of Tom Stoppard's play at the Gate Theatre, in February 1999, a mobile telephone rang garrulously in the auditorium while the 19th-century tutor Septimus Hodge attempted to edify his young charge. Breaking from the script, Cooper treated the distraction as an annoying fly, zigzagging across the fourth wall, until the phone was switched off. For the rest of the night the audience was his.

"Doing live comedy makes you respect an audience," he says. "It makes you communicate in a way that isn't at all the way one would in a play."

For a similar reason, it's not too odd that he speaks about the subjects of his comic impersonation as characters. "I do find it interesting what makes people talk or act the way they do, and I suppose it's more from an acting point of view that I impersonate people." He claims never to research his impressions, with one exception. "One person I did work on was Pat Kenny." Indeed, he has Kenny down pat - although Murphy is having none of it. "I can't see Risteárd sitting down and working at all," he says.

It's not the first time the question of discipline has raised its head. "Ah, he's not at all disciplined," laughs Máire Cranny, Cooper's drama tutor during his school days, at St Michael's College in Dublin. Her recollection of him - "very charming, very handsome and great height" - remains unimpeachable. Amused by her comment that it's extremely hard to discipline talent, Cooper recalls his university drama tutor's crueller analysis: "You're not as good as you think you are."

The subjects of his evolving mimicry, from schoolteachers to soccer pundits to media mouthpieces, beg the question of whether he has a problem with authority. "I have a problem with people being in authority who shouldn't be. There are many people in this country who are in that position. And I have a problem with being told what to do by somebody that I don't have any respect for. When you enter this business there are so many people that need to be impressed enough for you to get work. I suppose in the last while work has generated itself for me, so I'm not so worried about what other people feel about me."

As something of a double agent, slinking between theatre and comedy, Cooper has been privy to quite different worlds, worlds that may feel differently about him.

"Sometimes the process of theatre can take itself far too seriously, and it needs to shake itself up a little bit. The preparation for a live comedy is obviously very different to a play. But I think both could benefit from a little bit of the other."

So the comic and the actor can be friends? "I know that actors have a reputation for being extremely bitchy, but in my experience that isn't really the case. Comics are incredibly bitchy. Comics are vicious. And usually they can end a tirade of abuse with a laugh, so therefore it's funny."

After "charming", the word most employed to describe Cooper is "self- effacing". Handy for a mimic, one suspects, but perhaps less so for a man who must carry his current play almost single-handedly. Anna Manahan's role, while crucial, is almost completely silent, and the publicity for Auntie & Me trades heavily on Cooper's name, much as it did for Alan Davies's in its West End incarnation. With the same director, Anna Mackmin, and design, that production seems to have been imported wholesale. Only the names above the title have been changed. Although he emphasises the relationship between auntie and he as the core of the production, Cooper concedes that it is a star vehicle - "and I'm glad to ride on it".

Despite some imitable characteristics - expansive gestures, cautious, considered speech and an over-reliance on the phrase "to a certain extent" - it's unlikely Cooper would consider himself ripe for impersonation.

Given his habitual self-effacement and the elasticity of his persona, it's some relief that he appears annoyed at a misspelled production flyer, on which the fada on his name had slipped onto the wrong vowel. The leaflet has since been reprinted.

So how well does Cooper know himself? "I think pretty well at this point. I probably didn't for a while, but . . . there are boundaries in place now, whereas before there weren't really. To use a drama-training type word, I wasn't very centred. But now I think I am."

Barry Murphy might agree. "I think he'll always be guided by the discipline of the acting profession. I couldn't do what he does. I just don't have the discipline for the months of rehearsal and performance of the same text. He's very ambitious and he doesn't look for short cuts."

A single yellow Post-it note clings to the wodge of text Cooper will soon be enacting. It bears the five lines out of 50 pages he has yet to nail. He practises one on me, in which Kemp recalls watching an old man limping across a park. It is sensitive and mordant, thoughtful and impish.

"I imagined the effort it must have taken him to get there. The physical pain of losing that limb, the interminable rehabilitation he must have undergone, the taunting of children, the personal anguish and torment, just so he can walk across the park clutching his little bag of groceries. Why? Is life that important, that no suffering, no humiliation, is too great?" Cooper pauses for effect. "I just wanted to push him over."

Auntie & Me opens at the Gaiety Theatre, Dublin, tomorrow