In Scotland we know him as a brace of villains. To have sung Alberich in the Ring and Pizarro in Fidelio, and done so with such relish, has earned him a reputation difficult to shake off, especially as he has also sung Scarpia in Tosca 150 times. But Peter Sidhom does not regard Wagner's deranged dwarf, Beethoven's baleful prison governor and Puccini's sadistic police chief as type-casting. He has done other things, too, including a spot of Gilbert and Sullivan, though his malevolent Dick Deadeye in HMS Pinafore hardly seems to warrant his insistence that comedy came first in his career as an opera singer.

In Glasgow next Tuesday, however, the Egyptian-born baritone will bring to fruition his long-cherished ambition to portray Verdi's Falstaff. A dark and brutal Falstaff perhaps? Even sung by Sidhom this seems unlikely, and he seems to welcome this glowing escape route from the works with which we have come to associate him. Well, then, perhaps a lean and hungry Falstaff? That, too, sounds improbable, despite the distinguished music critic Martin Cooper's famous claim that Falstaff should never be fat.

Though not even Falstaff is immune to conceptualised productions, maybe the only concession to modishness made by Scottish Opera's latest presentation - by Dominic Hill of the Traverse Theatre - is updating Verdi's comedy from Shakespeare's time to 1893, the year in which it was composed. Though hardly a novel idea by current operatic standards - think of how the Edinburgh Festival last year transferred Strauss's Capriccio from eighteenth-century France to the time of the Nazi occupation because that was when it was written - this is an interesting decision which releases Verdi's Falstaff from the Elizabethan costumes in which it is traditionally enveloped, and so often seems to reduce the bite of the music. Though not quite so radical a time-switch as that of the Salzburg Festival's recent and fascinating Strindbergian Marriage of Figaro, it will unquestionably alter many aspects of the opera's established perspective.

Sidhom, at any rate, says he likes it, not least because it will allow him to retain Falstaff's fatness, which he also likes. He has even been practising a rocking gait, necessitated by the layers of clothing, and the mould of thick cotton padding reaching down to the top of his legs, which he will have to wear ("It's going to be quite warm," he suspects). But Falstaff's paunch, he maintains, is so much a part of him that he cannot imagine playing the role without it, even if it adds to the difficulty of articulating Verdi's racy and resonant music. The way he fondles the thought of the fat knight's girth, indeed, makes it sound as vital an object of desire as Alberich's ring. As he puts it: "Falstaff loves his belly. It's almost another character in the opera. He calls it his kingdom and he revels in it, saying that if he loses weight he won't be himself any longer." If today's trends in obesity require any explanation, here it is.

But, in other respects, has Falstaff in rehearsal been proving the experience he hoped for? All and more would seem to be the answer. Though some people continue to call Verdi's last opera a connoisseur's piece, too restricted in its appeal ever to lure the general opera-going public, and too fleet in its action to include the big tunes always expected of the composer, this is precisely where, for Sidhom, its attraction lies.

It's not that the work is so fast that it races past its audience. In fact, it is the richest of Verdi's scores, as a good performance makes plain. Even Stravinsky's gibe about its lack of arias - it was, he remarked, "poisoned" by the cult of Wagnerian music drama - no longer seems relevant, though one can see Sidhom being amused by it. The point, in any case, is that the entire work is an aria, a single non-stop tune or flow of tunes.

For all Falstaff's fatness, moreover, Sidhom considers him a dapper figure, and this, one imagines, will influence the way he plays the role. As he seems well aware, many productions of the opera founder on the fact that the comedy can be grossly overplayed, not only by Falstaff himself but by other members of the cast. I myself have seen productions wrecked by the exaggeration of every detail, particularly what can seem the hugely contrived search for Falstaff inside the laundry box, with clothes being flung all over the stage while the audience is able to observe perfectly well that he is not inside.

Whether this will happen yet again next week remains to be seen, but Sidhom is right to say that "if you consciously try to be funny in Falstaff it doesn't work." The music speaks for itself. The silent downbeat with which the work opens, and the crashing chord and fizzing arpeggio that succeed it, set the tone and rhythmic vitality of the occasion which no quantity of sight-gags can replace.

Written when Verdi was 77, Falstaff reveals the composer's genius not only as undimmed but also as capable of cocking a snook at his operatic past. The flourishes of brilliant self-parody in the work, as Sidhom points out, are just some of its special pleasures. Ford's sombre jealousy monologue recalls Renato's fateful Eri Tu in A Masked Ball and any number of other arias of its sort. Falstaff's counting of the midnight chimes in the closing scene evokes Rigoletto.

Yet there is also a genuine pathos in Falstaff, though the fact that Sir John is finally willing to forgive and forget what has happened to him is reflected in the triumph of the final fugue. The work, Sidhom proclaims with delight, is the glorious fulfilment of all Verdi had done before.