Maggie Smith: A Bright Particular Star, Michael Coveney’s 1992 biography of the actress, took his subject to the age of fifty-seven. She had just become a dame and was about to play, after some hesitation, Lady Bracknell in The Importance of Being Earnest, the role that Dame Edith Evans had once made her own. The Smith of Coveney’s first book was, like Evans, reclusive, preferring to stay in the country with her husband Beverley Cross and her two sons from her ill-starred marriage to Robert Stephens. This retitled edition, 150 pages longer, brings us to Dame Maggie at eighty, widowed, adored, familiar with a dowager costume and as reticent as ever. (Evans, if asked about her husband, would retort, ‘It is not of the slightest interest to anyone but myself, to know to whom I am married.’)

Capturing a living subject without the aid of papers, diaries or letters can be a deadly pursuit. Theatrical biographers resort to ploughing through reviews and synopses of plays and films. Some of these can be illuminating (‘She looks as crisp as a celery stick and speaks like a girl who has a good mind of her own,’ declared C A Lejeune in 1957), some risible. In 1959 Bernard Levin embarked on a lifetime of adulation when he said that Smith gave a ‘champagne-bubble performance’ in William Congreve’s The Double Dealer as Lady Plyant, and that his eyes ‘curtained with tears of joy’ at her Rosalind, her voice ‘like a chime of golden bells’. All praised her comic timing; all noted that she flapped her wrists.

Coveney proposed himself as biographer in 1990, knowing others had been spurned but emboldened by a geographical coincidence. Like her, he was born in Ilford and like her, he fetched up in Oxford. He went to Worcester College; she had been at Oxford High School and then in rep at the Playhouse, where she was seized by bright undergraduates, including Ned Sherrin, to adorn their revues. Her response to Coveney, as to everyone, was, ‘How absolutely ghastly … I can’t think of anything worse. There’s nothing to write about. I haven’t done anything.’

This line has the virtue of sounding unmistakably like her, a rare moment in this book. Another is her telling Alan Bennett that when Stephens ran off with the make-up artist on The Merchant of Venice, ‘the quality of mercy was pretty strained’. She also told an interviewer that getting back together with Cross, whom she should have married years before, was ‘like a script. The kind of luck that’s too good to be true.’ ‘Like a script’ reflects her lifelong preference for the theatre over real life. ‘I’m never shy on stage. Always shy off it,’ she confided to Nancy Banks-Smith. ‘Theatre is a different world. A much better world … It’s strict. It’s secure.’

If only Banks-Smith and Smith had collaborated on a book. I can’t imagine Banks-Smith pointing out, as Coveney does, Smith’s splendid breasts, or claiming that the piggery farce A Private Function was possibly Maggie’s best film, ‘one of the funniest and most nearly perfect British films of the century’. Coveney also finds The Pumpkin Eater (in which she played Philpot the nanny) ‘depressing’ and somehow knows that ‘the physical side of life was fairly unimportant’ to her. Did Stephens tell him that? She remains ‘a lax correspondent to this day’, Coveney notes in a peevish aside, ‘and no great emailer either’.

But she clearly approved Coveney’s earlier book, because the 1993 paperback edition contained a ‘postlude’ describing lunch with Maggie and her family in West Sussex, at a pub famous for fish, where she tucked in like a trencherman. In the intervening twenty-two years Coveney has become even more combative on her behalf, admiring her scorn for the bullshitting hyperbole of the showbiz world and her disinclination for party-going, party-throwing or attending award ceremonies. If anyone comments that on stage she’s always Dame Maggie, Coveney is liable to become pugnacious: ‘The charge that an actor is always the same from role to role is self-evidently crass.’

Smith takes risks, doesn’t mind being unsympathetic and has no trouble playing the lonely Judith Hearne or Miss Shepherd, the crazy titular character in Alan Bennett’s The Lady in the Van. Smith plays Miss Shepherd ‘without a scintilla of sentimentality’, making dangerous weapons of her ‘witty elbows’. And so she remains in demand, along with her fellow dames Eileen Atkins and Judi Dench, appearing in movies such as Ronald Harwood’s Quartet and, of course, in the Harry Potter films: prepare for pages of plot synopses. The new book is poised to catch the Downton Abbey market as the series reaches its end. Her Downton performance, says Coveney correctly, is ‘basically a piss-take’. So is her approach to biographers; she keeps her secrets still.