It was almost invisible, lying lost in a corner, soon to be discarded. But a visitor to the set of the Abbey Theatre's recent She Stoops to Conquer could appreciate its detail. It was a button, made of gold-coloured metal with an inset burgundy-red stone. It was heavy in the hand. To anyone who had seen Marty Rea's performance in the show, which had closed the previous night, it looked familiar.

Rea had played Young Marlow, Oliver Goldsmith’s dashing gadabout, who can revel with the scullery maids but becomes a gibbering wreck beside ladies of elevated status; it’s a role that must be played with ferocious energy. For its three-hour duration, Rea kicked up his heels or cowered, swaggered or stuttered, and often brought the house down. It may not have been his finest role, but, alive to every detail, he was the finest thing in the show.

"Those feckin' buttons were the bane of my life," says Rea, as we cross the street from the Gate Theatre, where he is nearing the end of rehearsals for The Caretaker. Rea, a charming and gracious speaker, is surprisingly exercised. "They'd fly off my costume and on to the set or into the auditorium almost every show." As Rea exited a scene he would inform the stage managers about another casualty. His costume, which he praised Joan O'Clery for designing, contained buttons delivered specially from France and which were hard to replace. They mattered.

"Marty has been asking the right questions from day one," the director Oonagh Murphy told me last year, when they were working together on Druid's production of Brian Martin's Be Infants in Evil. He has a reputation as a performer of confidence and rigour.

"It was like being one person in the day and being a completely different person at night," Rea says of his most recent overlapping roles: rehearsing The Caretaker, in which he plays the lumbering innocent Aston; while performing in the whirligig comedy of She Stoops. "I know that sounds ridiculous but that was exactly how it felt. Your head wasn't as busy as your body in She Stoops to Conquer. And during the day my head was much busier."

There is a productive tension in such dissimilar parts. Just last year, he was a leading man (for Druid, the Gate and the Abbey) in contemporary dramas and costume comedies. In the same year, he played supporting roles, such as Fr Kilgarriff in Tom Murphy's recent Brigit for Druid (a company for which he has now played three priests, each unlike the other). To hold such distinct characters in his mind, often at the same time, might be the mark of his background, as a working-class kid from west Belfast for whom acting was another world, or his remarkable training: when Rea was 20, he received the Lady Rothermere Scholarship Award to study in Rada.

‘We’re not just puppets’

“I think the really important thing is we’re doing this,” he says, clarifying a question about an individual approach to performance. “I’ve always maintained that my training is as important as anybody else’s in this business. The actors aren’t just puppets to be moved around the stage.”

He has clashed with some directors over this, he says. “There’s a generation of actors still going now who consider the director to be a nuisance. You know Edith Evans’s quote? ‘Who are you?’ ‘I’m the director.’ ‘Well you sit over there and we’ll call you when we need you.’ That’s very old-fashioned and I don’t think it’s very fruitful. Nor do I think the director has become the all-important thing now. I think better work happens when both are pushing each other, that this is a joint effort.”

He speaks with particularly high regard for the ensemble Garry Hynes built to perform DruidMurphy, a bruising and riveting trio of Tom Murphy’s works. “We were such a united front with Garry, but also we were united when she was pushing us in ways we were afraid of. You are like an army. It’s definitely for a bigger cause.”

Rea will talk about performance in almost nano-detail (when he spends a few minutes explaining various ways he might encourage me to drink a glass of water – helpfully, condescendingly, placating – it’s like building up a picture in pixels). Actors aren’t always so articulate or tenacious about their craft. Is this a product of his background or a skill learned in training?

"I think it's both things," he says. "I wouldn't have been able to articulate it until I trained. Even after I trained, I was afraid. I kept all my work very, very personal. Homework-based. It used to be an embarrassment. People would say, 'Oh you'll have no problem now with a Rada training.' That wasn't the case. In fact, sometimes Rada can be a problem, because people consider you to be a certain type." An RP-accented, elaborately costumed, Henry V type? "That's right. It was weird. Because I was also Irish. English casting directors didn't know what to do with me."

His first instinct, on returning to Ireland, was to conceal his training. Now, in conversation, he refers to it frequently. It was, in many ways, the making of him.

A painfully shy kid

Rea was a painfully shy kid. He stood out, a very tall and skinny boy who sprouted early (he’s now 6ft 2in) and was a target for bullies. He runs through his misgivings: “My accent, my voice, my posture really.”

In a Shakespearean scene study at Rada with the tutor Barley Thomas ("She taught me the whole idea of 'we're in this together' "), it all came to the surface when he was playing the regal Leontes in The Winter's Tale. Thomas encouraged Rea to behave as though he owned the room, the land, the people, and laughed at his attempts. Rea insisted he was. "Then why are you talking like this?" she asked. Rea tilts his head to his shoulder, slouches low and clasps his hands loosely over his waist. "It hit me: I've spent my whole life talking like this and listening to people. It was like a punch to the throat. I was really offended by it. Then I was worried about it. Is that what everyone thinks? Have I been just a pushover the whole time?"

There's a reason people call acting "the shy man's revenge". Today, Rea cuts a confident figure, even when playing unconfident people. He stands tall and dresses so impeccably – today in a grey tweed suit and flat cap – that I'm reminded of an exchange from The Wire: "There's a name for people like that," sniffs McNulty, a cop, as he and his partner spy on a dapper man. "Yeah," says Bunk: "A grown-up."

He readily traces his professionalism to his origins, concealing nothing. It was his English teacher, Dermot Campfield, who brought Shakespeare alive in a classroom in St Mary’s Christian Brothers School, suggesting Lear’s Goneril and Regan as women “in big knee-high leather boots – they’d kick the shite out of you”. Rea laughs. “All of a sudden I could imagine them. Coming from the Falls Road, I can picture those kind of women very well.”

His father, Charlie, instilled discipline in him. “He always maintained: if you’re going to do a thing right, do not do it by halves. It has helped me and driven me through this business. Like the button thing. If a button fell off my coat, I will make as much of a fuss about it as anything else. I’ll sew it on myself. On the last night, when I know there are people going, ‘Oh, don’t worry about that, it will be grand’, and I know it is grand. But if we start saying ‘it’s grand’, when do you stop? I think Ireland and the Irish will suffer from an ‘it’s grand’ mentality.”

This, you feel, is typical Rea: to begin with the smallest detail and work up to the big picture. It’s a simple instruction and a hard-won philosophy: button it.

The Caretaker is at the Gate until March 21st