It’s all go for Judi Dench, stuck at her house in deepest Surrey. What a freewheeling week; she is beside herself with excitement. Yesterday, she explains, she received her Covid vaccine. This required a trip to the village and was the first time she had left home since she can’t remember when. Then today it’s a phone interview, the thing she is doing right now. Her cup runneth over. Her world has turned Technicolor. “I’m not even joking,” she says with a sigh. “It’s nice to actually have something to do.”

Lockdown, I fear, is not the life Dench was born to. She used to practically eat and drink on the stage, but the theatres have closed, who knows for how long. She used to bounce from one film set to the next, but now production is mothballed and the industry has gone to ground. All of which means that she is confined to the house, an 86-year-old actor shoved into what she hopes is a partial and temporary retirement. She gets up each morning determined to keep herself busy. She crawls back to bed with most of the tasks left undone. After a while, she admits, the time starts to drag.

Dench recently learned a new word: synesthesia. “And I thought; ‘Well, that’s me.’ Because I always saw the days of the week in colour. I never gave it a second thought, it’s just how my mind works. And all of a sudden it’s not there any more. The days of the week have no colour at all. There’s no structure, no planning.” She is marooned with her memories and mementoes and various unquiet ghosts.

As luck would have it, her most recent film similarly throws her in among ghosts – although here, again, the experience soon starts to grate. Blithe Spirit is a galumphing reanimation of Noël Coward’s 1940s farce, played with gusto but fatally heavy-footed. Dench co-stars as Madame Arcati, a preposterous old medium who was previously embodied by the likes of Margaret Rutherford and Angela Lansbury. Down the years we have grown accustomed to seeing Dench making herself blissfully at home in any film, big or small, but her role as Arcati feels like so much heavy lifting. She huffs and she puffs. She falls into the orchestra pit. If the film is a notch or two up on 2019’s calamitous Cats (in which she played Old Deuteronomy), it is still a far cry from the heyday of Philomena, or Notes on a Scandal. Blithe Spirit is running on vapour, shouting to be heard. In the end it is a bit of a ghostly presence itself.

Dench says that in real life she probably has seen a ghost. She is reluctant to discuss it. People will think she’s gone daft. “But I remember being at [the actor] Michael Dennison’s memorial, which was at the Haymarket theatre, early in the morning. And I was walking down the stairs to the stalls and saw somebody in a black tailcoat run down in front of me. And then at the bottom there was nobody there at all.” She pauses. “But a lot of people say they’ve seen ghosts at the Haymarket, or at theatres all over. Ralph Richardson was certain that he did. And it makes perfect sense to me. There’s always a lot of spirits in the theatre, I think.”

Dench spent decades haunting the theatre herself – first in her home town of York, then at the Old Vic, then all over. These days, she tires more easily: an extended run can feel like a marathon. Also, her eyesight is almost completely shot (macular degeneration; she doesn’t want to harp on about it). But before that, good heavens, there was no stopping her. Directors knew her as “the Mighty Atom”, a 5ft 1in ball of fire, versatile and enduring. She says that when she wasn’t on stage, she would hang about in the wings, simply watching the show, drinking in the experience. She loved the interaction with an audience, the endless possibilities; the sense that a play could change and grow over time and that no performance was the same as the last.

It’s ironic, she says. She never especially wanted to be a film actor anyway. “Recently an American journalist said to me, ‘Oh, and I believe you did a little theatre as well’, and it was like an arrow going through my heart. I thought: ‘Oh dear, there goes the 60s, the 70s, the 80s, the 90s.’ Thousands of performances, gone in a flash.”

Besides, she adds, it’s not as if Hollywood was knocking the door down. When she was first starting out, a well-known film director turned her down for a role. He said that she was altogether too short and too plain. She should stick to the stage and leave cinema to the thoroughbreds.

I have heard this story before, but I don’t believe she has ever named the culprit. “No,” she says sharply, like a schoolmistress scolding an unruly pupil. “I never have and I never will. I’m not going to break my word today. The film in question, though, went on to be enormously successful. And I’m not going to tell you the name of the film, because then you’d know who it was right away.”

In which case we’ll move on from one louse to another. Dench’s film career is her own handiwork. She is a peerless performer; a commanding screen presence, as comfortable in a Bond film as she is in The Best Exotic Marigold Hotel. But her Hollywood breakthrough was engineered at least in part by Harvey Weinstein, who is serving a 23-year prison sentence for third-degree rape and first-degree sexual assault. It was Weinstein who spearheaded the awards campaign for 1998’s Shakespeare in Love. In her role as Elizabeth I, Dench was on screen for all of eight minutes. She went on to win the best supporting actress Oscar.

I ask whether her association with Weinstein has retrospectively tainted the experience of acting in the film and winning the Oscar, and she insists that it hasn’t. The film’s still the film. She loved making it. That hilarious costume she had to climb in and out of. “But I worked a lot for Harvey, a huge amount,” she admits. “And he was always completely charming. Perhaps I was lucky, but that’s all I know. I feel very acutely for the people who weren’t so lucky.”

Does she feel she misjudged him? She takes a moment to consider this. “Misjudged him?” she says. “I don’t know that, Xan. He was a friend. He was a perfectly polite and funny and friendly person. I never experienced Harvey in any other way than that. I knew nothing untoward about him at all. And nor was I warned. So of course I can judge him. But I never experienced that other side of him at all.”

The line’s breaking up. We are struggling to make ourselves understood. “It’s as if you’ve gone underwater,” she shouts, although maybe this is a blessing. If Dench’s career has taught her anything, it is to stay fresh and keep moving. Wallow in the past for too long and the ghosts might drag you down.

She feels that the secret to lockdown is to keep busy, stay fresh. But that is easier said than done. For the past year, she says, she has been sharing the house with Sam, her 23-year-old grandson. They hang out together and make TikTok videos. But they are basically killing time until the world starts up again. Sam had a job all lined up with the theatre producer Bill Kenwright. Now he is sat on the sidelines twiddling his thumbs; him and thousands like him. If lockdown is tough for her, it is doubly so for Sam.

The problem, I suspect, is that Dench is a workaholic. She once claimed that she says yes to everything, never turns a gig down. I assumed this was a joke but she insists it is, by and large, true. Her agent reads the scripts first, weeds out the bad ones and provides a quick precis of what the good ones are about. After that, more often than not, it is simply a case of rearranging her diary.

What would it take to make her say no to a film? What is the one movie role she would totally balk at playing? “Probably an 86-year-old woman who’s not able to see.” She laughs shortly. “I’m not going to play that part, thanks.”

Ideally, she’d prefer some darker material. One of her favourite film roles was that of Barbara Covett, the vicious, miserable schoolteacher in 2006’s Notes on a Scandal. It would be nice to have the chance to make another picture like that one. “But I get put in a bracket. ‘Oh, would you like to play this person who’s sitting in a care home and she’s in love with a bird? And then she gets upset because, I don’t know, something happens to the bird.’”

Here, perhaps, is the downside of Dench’s film success; the consequence of being a part of the nation’s collective cultural fabric. We prefer our brightest stars to remain in one place. It is reassuring and soothing; it is how we plot our course through the world. When they start zigzagging about, it confuses the public; makes us lose our bearings.

Dench claims that she especially detests being described as a national treasure and who can blame her? It’s reductive and condescending. It puts lead boots on her feet. Others, though, have suggested that perhaps she doth protest too much. The late Geoffrey Palmer, her co-star in the BBC sitcom As Time Goes By, swore blind that she only pretends to dislike the term. “[That] would be an absolute bloody lie,” he said. “She loves it.”

I put this to Dench and she splutters with righteous indignation. That’s Geoffrey, she says, teasing her from beyond the grave, making her justify herself all over again. Because it is true that she hates it. Always has, always will.

“For one thing,” she says, “it’s a terrible label. So dusty, so dreary. For another, it relegates me to being an 86-year-old woman. Whereas in my mind’s eye I’m 6ft and willowy and about 39.”

She takes a breath and mulls it over. She says: “Do you know what it’s like? It’s like they’ve picked me up and put me inside a little glass-fronted cabinet. Then they’ve locked the door so I can’t get out.”

The line is breaking up. We have gone underwater again.

Smash the glass, I tell her, raising my voice to be heard. Smash the glass and run amok.

“Exactly,” she says. “And fuck being 86 years old.”

• Blithe Spirit is on Sky Cinema now