"Anyway, that's not my problem, is it? That's the Lord's problem."
She was referring to Lord Fellowes, creator of Downton, who said earlier this year that he had already begun writing the script. "I've done some work on it because I don't want to be caught out if [the producers] suddenly say yes and then it's all go," he explained.
Dame Maggie, 82, said she could not envisage the Dowager in a film, unless it began with her funeral. "I could croak it, and it would just start with the body," she said.
Cast members who have indicated they would like to appear in the film include Phyllis Logan, who plays Mrs Hughes, and Jim Carter, aka Carson the butler.
Appearing at the Radio Times Television Festival, Dame Maggie said Downton had changed her life - and not always for the better.
"It's ridiculous. I led a perfectly normal life until Downton Abbey," she said. "I'm not kidding. I would go to theatres, I would go to galleries and things like that on my own. And now I can't. And that's awful. [Even] the Fulham Road's dodgy.
"And it's all through television. I had been working for a very long time before Downton Abbey and life was fine, nobody knew who the hell I was."
Her Downton character is famed for her one-liners, and was moved in one episode to ask: "What is a weekend?' Dame Maggie said: "Last night a Brazilian lady asked me to say, 'What is a weekend?' I thought she was seriously asking me a question."
Her role as Professor McGonagall in the Harry Potter films also brought her a new level of fame. Asked about her young fanbase, Dame Maggie said: "They are charming. Truly. If they want an autograph or a photograph - fine. It's the big people who should know better."
One of the few actresses to have won an Academy Award, a Tony Award and an Emmy - in fact, she has won two Oscars, for California Suite in 1979 and The Prime of Miss Jean Brodie in 1970 - she joked about her rivalry with a fellow dame, Judi Dench.
Asked about the dearth of roles for older women, she laughed: "Judi's always there. It was Joan Plowright who said that to me: there are always parts, but Judi gets her paws on them first."
In the hour-long talk, Dame Maggie reminisced about many of her film roles, including The VIPs, in which she had a scene with Richard Burton in London Airport. "It was a very odd scene to do," she recalled, "because he had a bottle of whisky which I was deeply shocked to find out was a real bottle of whisky. And quite a lot of it went down the little red lane. But he was very generous in that scene."
She was asked, given the fact Sir Ian McKellen once took a role in Coronation Street, if she would ever appear in EastEnders. After a pause worthy of the Dowager Countess, she replied: "I'd rather not."