The playwright Terence Rattigan once informed Harold Pinter that he knew exactly what The Caretaker was about: "God, the Holy Ghost and mankind."

"No," Pinter replied wearily. "It's about two brothers and a caretaker."

When David Bradley first read this jarring modern classic at drama school in the Sixties, he thought he might like to play one of the brothers: Mick, perhaps, alternately brutal and benevolent with a fascinating flicker of sadism, or his dispossessed older brother, Aston, made sweet and slow by electric shock treatment.

"You never think," he says, "that you're going to be old enough to play Davies".

In conversation Bradley is warm and playful and can take on the soft RP of his mentor, Laurence Olivier, or the rhythmic vulgarity of his heroes Pete and Dud with uncanny accuracy.

But at 64, with long, grizzled hair, granite features and a hardening Yorkshire accent, the RSC stalwart seems to be making unconventional caretakers his particular speciality.

Since 2001 Bradley has become known worldwide as Argus Filch, the misanthropic caretaker of Hogwarts school in the Harry Potter films. And he is now starring alongside EastEnder Nigel Harman and Telstar's Con O'Neill in this touring production of The Caretaker, playing Pinter's manipulative, belligerent and often grotesquely comic tramp.

Pinter's 1960 breakthrough play was based on real events which occurred after the playwright, his wife and newborn son moved into a two-room flat on Chiswick High Road.

A kindly man named Austin looked after the other flat in the house for his brother, and one day brought home a tramp he'd met in a cafe.

Pinter's imagination was sparked by the curious mixture of loneliness and aggression displayed by this unexpected guest, and by his last image of the pair, glimpsed through an open door: Austin, his back to the room, was gazing silently out to the garden. The tramp, apparently given his marching orders, was stuffing his meagre belongings into a grubby holdall.

"He must be the most cantankerous, selfaggrandising, shifty geezer in modern drama,"

says Bradley of the vagabond's stage incarnation, Davies. "He's been on the road for most of his life, it seems, and this is his last opportunity to find a bit of warmth, a bit of comfort. He will use any devious means to achieve that, but there's nothing really complicated about him.

"He's just a survivor, and no matter how obstreperous he can be, I admire his tenacity."

In Pinter's subtle, disturbing and brutally funny play, Aston, in need of a confidant, offers Davies the rather indistinct position of caretaker to the cluttered West London attic. To secure his position, the tramp begins to play off one troubled brother against the other - but reckons without their fraternal love.

"Imagine when this first hit the stage," says Bradley, himself a working class ex-factory worker who had never heard of RADA until he was advised to audition. "Everyone was used to all these three-act plays with French windows and gin and tonics on the veranda. People with working class accents were confined to being the servants or gardeners.

"Theatre then was a comfort zone, a diversion.

To hear this crackling, aggressive dialogue from the main protagonists must've been quite a surprise."

Simultaneously lyrical yet tough, brutal yet rhythmically precise and full of menacing pauses, Pinter's style was so groundbreaking in the Sixties that it has become a theatrical adjective. But it's nothing, Bradley observes, that you wouldn't hear on the street.

Before rehearsals started for The Caretaker, the actor was driving back from Belgium with his wife and son when he spotted a sign for Sidcup.

It is to this London Borough, where he claims to have left his identifying papers, that Davies spends the play finding excuses not to return.

Curious, Bradley took his family on a detour.

"It was a sunny Sunday afternoon, and we didn't see anything that made me think, this is the world of the play' until we passed the Working Men's club," he recalls.

"Outside there was a whelk stall, and the lady was saying, Awlright darl, what can I do for yer? I ain't got the prawns today, only got the whelks, jellied eels and cockles.' "Then this guy in a van screeched off the main road into the car park of the club and shouted, Did you see that? Did you see that bastard? 'E signalled left, an' 'e didn't even turn left, the bastard. Why? Cos 'e was on 'is mobile phone, that's why'.

"I said, Well, course that's illegal now', and he says to me, I don't care if it is illegal, he shouldn't be doing it!' "I thought, my god, that's Davies talking."

When Pinter was awarded the Nobel Prize for Literature in 2005, the Academy declared him a playwright who "uncovers the precipice under everyday prattle and forces entry into oppression's closed rooms".

A grand statement that is difficult to appreciate the validity or value of - until Bradley describes some of the audience members who came to the production's opening run at Sheffield's Crucible last October.

There were people who had had electric shock treatment in their youth who wept through Aston's monologue, he says. And there was a group from the local homeless centre.

"They're still sleeping on the streets at night but go to the centre for a bit of warmth and a meal,"

says Bradley. "They'd never been to the theatre in their lives before and they said, That's my life up there'. One afternoon me, Con and Nigel went and spent some time there. There was a guy called George who is always going on about shoes, just like Davies.

"One guy said to me, You must've spent some time on the streets then, have you? Where were you?' I says no, I'm just an actor.

"They said, We thought you were one of us.'"

In the coming months, Bradley will be appearing as "a terrifying Manchester paterfamilias" in black BBC comedy True Dare Kiss, and reprising his role as "a terrifying Manchester gangland boss, head to foot in leather with a voicebox" in Johnny Vegas's Ideal.

He also has a cameo in Shaun Of The Dead follow-up Hot Fuzz, as "a psychotic West Country farmer with a shed full of incomprehensible weapons".

Most eagerly awaited, of course, is the fifth Harry Potter film, released this July. In Harry Potter And The Order Of The Phoenix, Bradley forms a double act with Imelda Staunton, who plays Professor Umbridge, the Thatcherite new head of Hogwarts.

"Filch loves her because she's as strict as he is,"

says Bradley. "I had a lot of fun up a 35ft ladder, clinging on with one hand and hammering proclamations into the hall wall with the other, while four crew guys shook the bottom. I'm not very good with heights."

Now able to step into the character of Argus Filch in the time it takes to put in the fake teeth and hair extensions, Bradley originally went for the role at the suggestion of his children. A previous bid to raise his playground profile, when he auditioned for the part of a French villain in Superman, failed on the grounds of his French accent. ("I 'ave ze pow-air," he offers, tentatively.

"See, not one of my best.") "I said to the kids, What part do you think I should go up for?' and they suggested Filch. I was a bit disappointed. At least Snape is suave.

"That's a bit of a wake-up call, learning that's how your kids see you."

On accepting the role, Bradley was prepared to become a hate figure for children everywhere.

Instead he receives fan mail from Australia to Singapore and is often approached in the street and asked to croak one of Crouch's unlikely catchphrases into a mobile phone. "I always oblige," he says. "For someone of my tender age it's very nice."

With four cats of his own, Bradley also enjoys a good off-screen relationship with Mrs Norris, Filch's feline sidekick, played by two Maine Coons named Max and Alanis - "as in Alanis Morrisette", he says of the latter, with a sniff. "Bit of a poncey name for a cat isn't it? I hope she doesn't get abuse for it in the cattery."

The cats are "grunged up" with hair wax and fur-pieces to make them look scraggy, and Bradley uses treats to get them to follow him or jump up on his shoulder. "It often takes a few takes to get the cat to jump up at the right angle," he says, "so that its backside isn't covering my face."

The Harry Potter films have been a chance for Bradley to reunite with many old friends, among them Michael Gambon, Richard Griffith and Alan Rickman, with whom he started at the RSC in 1978 ("he seems to have got better looking, somehow, as he's got older.") "It's nice when you get a tea break, and we sit around among all the dusty cables behind the set for Hogwarts Hall," he says. "They put a special heater in so we can read our papers and have a gossip."

But of all the great British actors with whom Bradley has worked, none has had as great an impact on him as Olivier, whose Old Vic he joined in the early Seventies. It's strange to imagine Bradley as an inexperienced youth, overawed by his mentor and, one supposes, cleanshaven.

"He had me in his office once when I was doing Trofimov in the Cherry Orchard," he recalls. "Because I was from the North I was doing it like myself, not putting on too much of a posh accent.

"Olivier said, Oh, David, about this Yorkshire accent for Trofimov, why are you playing him as working class?' So I showed him the bit of the script where Trofimov mentions that he's the son of a grocer, and he said, I'm so sorry, I'm so sorry, I totally missed that - carry on'.

"I saw him a few days later in the corridor and he said, Darling, I'm so sorry about the grocer, terribly sorry about the grocer.' He'd say it every time I saw him, and it got the point where I was really embarrassed at having to keep saying it was all right.

"After a few weeks,"

continues Bradley, his voice hoarse with suppressed humour, "I realised that he was winking with every apology. He was just taking the piss."

  • Starts 7.45pm (matinees Thu and Sat 2.30pm). Tickets £15-£25. Call 08700 606650.