Imperious Smith rules the castle on ‘Downton Abbey’

“Downton Abbey,” 9 tonight, PBS.

There are now many reasons to watch surprise mega-hit “Downton Abbey,” which begins its third season on PBS tonight. But before the first episode aired, there was really only one: Maggie Smith.

Some critics and public broadcasting aficionados may have claimed allegiance to creator Julian Fellowes, the actor-turned-writer who had worked with Robert Altman to give us “Gosford Park,” or even a nostalgic delight at the return of Elizabeth McGovern. But if we are truly honest with ourselves, the main reason for all the early anticipation and adulation was Smith, a performer of such consistent, elastic and unique fabulousness that, well into her eighth decade, she’s practically become her own genre.

Querulous class-sensitive companion? Autocratic aunt? Snobby but possibly sensible duchess/countess/queen? Over the years, Smith has played the dithering and the withering, the brilliant and the mundane. She has Oscars for her unconventional schoolteacher (“The Prime of Miss Jean Brodie”), and wry and brittle movie star (“California Suite”) and more Emmy and British academy awards than is possibly seemly.

In recent years, she has specialized in sharp-tongued, perpetually appraising women whose implacable gaze and scathingly insightful retorts shield, one suspects, a heart broken young. Even before she began playing the redoubtable Professor McGonagall in the Harry Potter series, she could freeze a room with a look, orate in a perfectly timed silence, break your heart by simply squaring her shoulders or settling her shawl.

It was a big year for Smith, even by her perpetually prolific and wide-ranging standards. In “The Best Exotic Marigold Hotel,” she played a chronically dissatisfied former domestic worker without, initially, a kind word for anyone; in the newly released “Quartet,” she is an opera diva who brings her outsized sense of self to rest among a group of friends at a retirement community. She’s already received six nominations this awards season — four from the Screen Actors Guild and two Golden Globe nods.

In “Downton Abbey,” she is the dowager countess of Grantham, a woman who says whatever she thinks to whomever she pleases. In Season 1 especially, she was the most formidable, and entertaining, symbol of a class system collapsing under the weight of its own absurdity.

In her hands, sentences are as much sound effect, or in some cases, scores, as they are dialogue; she clatters and clips, slurs and simpers, dipping into this octave and that with the agility of an opera star. “Oh, Peter,” she sighs, as the grown-old Wendy in “Hook,” and suddenly that film is a lament for both age and youth.

Commanding presence

Like Angela Lansbury, whom the multitudes will also watch in anything, Smith is one of those women who has looked essentially the same since she was 20, whom age has burnished rather than diminished. In a face softened and unapologetically lined, those prominent blue eyes, that expressive slice of a mouth are even more arresting.

When she chooses, Maggie Smith remains one of an ever-diminishing number of women to whom the term “commanding” can accurately be applied. Americans have always been soft, but the modern relaxation of social strait-lacing has made its way to Britain, where “Iron Lady” Margaret Thatcher was replaced by People’s Princess Diana and now beloved commoner Kate Middleton.

As middle age expands to encompass pretty much any number between 40 and death, the ramrod stiffness of spine and expectation that once marked the elderly matriarch or dowager are quickly becoming antique, the rigors of posture no longer necessary in a world supported by Spanx. Smith, and a few of her peers, still have enough muscle memory to evoke these female pillars of Freudian nightmare and great literature, women of principle and control, who may actually understand the tyranny and contradiction of their particular social order but who believe that the chaotic alternative will be, in the end, much worse.

For better and worse, they are disappearing. These women ruled families without raising their voices; they ungently tried to prevent the follies of youth and considered snobbery not just a necessary sorting of the world but a sign of intelligence. With her double-edged smiles, quicksilver timing and easy service of backhand compliments, Smith not only keeps them vivid but also offers a master class to a younger generation of performers.