Ian Mckellen and Coronation Street

When Ian McKellen played Widow Twankey in pantomime last Christmas, becoming the first theatrical knight to turn into a dame, some critics commented on the resemblance of his drag acting to such northern matriarchs as Ena Sharples and Annie Walker. This now proves to have been a Freudian slip at the make-up table or a deliberate hint to casting directors because Sir Ian has signed up to appear this spring in 10 episodes of Coronation Street, playing writer Mel Hutchwright, author of the fictional Salford bonkbuster Hard Grinding.

Sir Ian was also one of four actors competing to appear yesterday in a special Radio 4 edition of The Archers, written by Victoria Wood for Comic Relief. Although he was beaten to the prize by Stephen Fry, this double assault on early eveningserials suggests that he's getting on his soapbox for soap or, at least, that the long-held snobbery about the genre is disappearing.

As in so many aspects of British theatrical acting, Sir Ian finds himself following Laurence Olivier. Larry apparently long fantasised about appearing in the Street and had the advantage in this ambition that his brother-in-law, David Plowright, ran Granada. A character role was scripted for him but an illness prevented Olivier from treading the cobbles.

Even so, a folk-memory of the connection has lingered because some websites insist that Olivier once secretly played a tramp in the background of a scene. And the fantasy of a classical actor guesting in peak-time populism has extended to other performers.

Michael Gambon has said his mother would only have believed that he had made it at the greasepaint game if he had turned up in a scene with Ena Sharples. Gambon sometimes seemed in such interviews to be fishing for the offer of a Salford walk-on but it sadly never came.

The explanation for Sir Ian's soap debut may simply be that he wanted to speak some proper dialogue after appearing in all that Tolkein trog tosh, but this benediction from a great Shakesperean actor to the Street almost certainly has a greater significance.

There's an obvious psychological impetus - McKellen comes from the north-west and, as with Gambon, there's a hint that this is the kind of acting that would have impressed the neighbours - and perhaps also a political agenda: the actor has complained in the past about gay representation in the show, and it seems likely that the character he plays will redress this balance.

Most notably, though, Sir Ian's equal billing with Ken and Deirdre signals an end to the stigma for stars in acting in soaps. Intriguingly, McKellen first agreed to appear in the Street some years ago but backed out before rehearsals.

It seems likely that his second thoughts then came from a fear of CV-contamination: the possibility that a scene between McKellen and Ken Barlow would be seen as the kind of joke about slumming it that was best confined to the plays wot Ernie wrote for the Morecambe & Wise Show. If so, it was a sensible concern.

However, McKellen, after successfully managing to alternate Gandalf with appearances in Strindberg and Shakespeare, seems now to have understood the extent to which cultural categories are breaking down. By the summer, he will have become, in six months, the first major classical actor to have appeared in both panto and soap.

The public and most pundits are impressed by promiscuity. These days, a sensible philosophy professor appearing on Desert Island Discs would always include the Simpsons theme tune between the Palestrina and the Satie. Two Oscar-winners have appeared in James Bond movies (Halle Berry and Judi Dench) and Sir Ian's Twankey was, in fact, a serious piece of acting, an exploration of a different type of comedy.

His Mel Hutchwright for ITV1 peaktime will hopefully offer the same pleasures of crossover acting. The one worry is that performers are notorious followers of fashion. So, after Sir Ian in the Street, we risk switching on EastEnders to find Dame Judi chin-wagging with Dot Cotton, while Sir Ben Kingsley chews over wool prices in Emmerdale.

They should be careful because McKellen is an unusually versatile performer, as anyone knows who has seen his Corialanus, Gandalf and Twankey. It's fitting that he should become the first star recognised on both Coronation Street and Sunset Strip.