It took less than a week of lockdown for Alison Steadman to start making puppets. Supplies weren’t a problem; this is a woman so anti-waste she thinks supermarkets should charge a fiver for plastic bags and donates her old hair to the birds. “It’s very good for nests; it’s soft and it complements the grass and sticks.”

So, come late March, she decided to knock together a Mr Punch to entertain her grandson on FaceTime. “I’d got all the stuff: toilet roll holder, newspaper, flour, plasticine, Christmas decorations, an old cushion.

“I love Punch and Judy. When I was a child, we’d sometimes go shopping in Liverpool city centre and my treat, if I behaved, was to watch it outside St George’s Hall. People say: ‘Oh, but he used to beat his wife with a stick.’ But as a kid you don’t know that. It’s just fun.”

Is her Punch around, I ask? Can I see? Ever obliging, Steadman disappears. Her iPad is propped on a coffee table, so when she’s not on the sofa, you see a watercolour of Holkham sands and some smoke detectors.

It’s a holiday home: tasteful, a bit nautical (her, too – blue tunic, white trousers). Steadman is a hardcore birdwatcher, at her happiest on the Isle of Man. “I hate beaches where all you see is people sunbathing and 1,000 umbrellas.”

After a minute, she’s back, soft and smiley as ever, along with one of most terrifying things I’ve ever seen. This is Punch after a hard apprenticeship in hell: wild eyes, Aztec paintwork, rictus teeth. Freddy, three, was a bit uncertain. “He prefers Sooty.”

The alarming darkness of Steadman’s creation – and the quality of craftsmanship (it’s brilliant) – shouldn’t be a surprise. But she is easy to underestimate. Or, perhaps, impossible to overestimate.

“She stands toe-to-toe with the greats,” says James Corden, whose sitcom Gavin and Stacey – in which Steadman plays clucking suburban mum Pamela – has brought her a new generation of admirers. The best actors, he thinks, are either aliens (Tilda Swinton, Mark Rylance) or humans. “Alison is always first and foremost a human being. Her lack of starriness or seeking status makes her so approachable.” It also means she can wrongfoot you.

Now 74, Steadman has two new leads as unassuming women you dismiss at your peril. In the undemanding drama 23 Walks, she’s a prickly divorcee whom Dave Johns valiantly tries to romance as they tramp round north London with their dogs. And in Life, a flagship BBC drama from Doctor Foster’s Mike Barlett, she is previously meek Gail, who has an epiphany at her 70th birthday party: her husband is a patronising idiot and she is not going to take it any more.

In the first episode, we are treated to some extreme closeups of Steadman’s face; the better, says Bartlett, to “capture in microscopic detail things that we haven’t seen Alison do before. You get it all: real nuance and the detail of a woman thinking two contradictory things at the same time. You can’t direct that. Alison is smart and truthful and instinctive enough to just do it.”

Sometimes, says Steadman, she was worried Gail goes “a bit far, was really mean”. But that was also the appeal. “I’ve never known a woman actually stand up and say: ‘Enough, I want things to change.’ Usually it just goes on until the end of life.

“My father was the kindest, most gentle man you ever met, but some friends of my parents had a relationship in which the husband would just put his wife down. My mother used to say: ‘How dare he speak to her like that?’”

Ingrained sexism endures, she thinks. “If you’re sitting around a dinner table and there’s two women over 60 and some gorgeous 30-year-olds, you can be sure that the older women won’t get noticed unless they say: ‘Excuse me!’”

You cannot imagine the friendly but forthright Steadman would stand for such nonsense. Corden remembers the first time they met, alighting from a train in Leeds to shoot Fat Friends, 20 years ago. “I asked if I could help her with her bags and she said: ‘Yes, you can, and don’t ever stop asking people that.’”

The only time she has suffered sustained bullying, she thinks, after a pause, was at the hands of another woman: a TV director, years ago. “I was just starting out, and she enjoyed humiliating me. It was horrible. When we’d finished shooting she bought everyone a drink except me. One of the guys on the show said: ‘Oh, she fancies you.’ I said: ‘If she fancies me, why doesn’t she make a pass?’ It was nonsense! She was just a bitch.’”

They crossed paths four years later in the BBC canteen. “She was all: ‘Oh, Alison! Hello!’ because I’d by then had a bit of success. So as she sat down, I just said, ‘Hello’, and got up and sat somewhere else.” Her voice shakes slightly. This seems to be about as vengeful as she gets.

Steadman’s breakthrough came aged 30, with the 1976 BBC film Nuts in May, written and directed by her then-husband, Mike Leigh, as was Abigail’s Party, shot the following year, when she was pregnant with their first son, Toby.

“I loved those early performances,” says Julia Davis, Steadman’s Gavin and Stacey co-star. “Because they weren’t as cartoonish as, say, Mrs Slocombe or as brittle as Sybil Fawlty.” (A friend of Davis wrote to Steadman on her behalf, saying she wanted to follow in the footsteps of her heroine; Steadman’s encouraging reply sealed the deal.)

“Fawlty Towers is incredible, and also has that sense of desperation, but it’s more manic and farcical. Whereas Alison’s characters in Nuts in May and Abigail’s Party feel like misfits who are trying to find love,” says Davis.

And, as recognisable grotesques, they are also ancestors of Davis’s own creations. “Beverly [her ghastly hostess from Abigail’s Party] is all kind of sexy and slightly sleazy but also weirdly cosy,” she says. It’s true: even at her shrillest – think Mrs Bennett in the BBC’s Pride and Prejudice – Steadman is somehow a comfort. “Alison’s genius,” says Leigh by email, “derives from her unique combination of being soberly sane and totally bonkers.”

Access to a reserve of battiness is hard to gauge on Zoom. But Steadman certainly seems more composed than most of the people she plays – “gloriously unaware and purely themselves,” says Davis. Yet she is also too interested in others to be especially self-conscious – she really misses going on the bus, she says, and wondering why someone is wearing those shoes.

She is also a natural, generous mimic – her impression of a woman recognising her in a pub is wonderful. She tells stories in dialogue, and is absolutely absorbed by domestic interaction. Her family – aunts, sons, Freddy – forever pop up in conversation. Just before lockdown, Toby cautioned her not to get the tube to a supper. “I said: ‘All right, we’ll get a cab.’ He said: ‘Yeah? Which company?’ But he was 100% right. In a week’s time, we dashed into Sainsbury’s, did a naughty big shop. Didn’t go back into one for the next four months.”

The exchange was similar to one in a short film, Unprecedented, made in May for iPlayer with her partner of 25 years, the actor Michael Elwyn, in which their offspring admonish them for continuing to socialise. Shooting today is strange, she says brightly, “but we adapt”. She can’t do plays for the foreseeable, but she had sworn off them anyway, after awful stage fright during her last one.

“I thought: ‘I don’t think I can do this again.’ Getting on stage has always been stressful. Then after a bit, you get your confidence and it’s the best job in the world. But last time I couldn’t get over that nervous state.”

On TV and radio, she averages about three projects a year, the majority for the BBC, which she is quick to defend. “It’s good to watch a drama without four adverts in the first 25 minutes. I definitely feel it’s undervalued. As with the NHS, people are like: ‘Well, it’ll always be there.’ But they both have to be nurtured.

“I’ve been really depressed about how the NHS has been left to flounder and they’ve sold off sections to the private sector. I mean, do we want to all have to pay £8,000 a year to Bupa just so we can get our toenails clipped?”

Maybe Covid will change things, she thinks. “I obviously didn’t want the man to be ill, but I was pleased Boris Johnson was suddenly talking positively about it.” She loved the clap for carers. “I quite miss that. Afterwards on the street we’d all have a distanced chat. I’ve got to know some of my neighbours much better.”

She wishes Keir Starmer was in charge but can’t bring herself to really criticise the Tories. “Everyday people like us, and the government – we’re all learning. I mean, I never thought that I would suddenly live in such strange and scary times.”

Steadman has got through by counting her blessings, moving her first gin and tonic half an hour earlier to 6.30pm and video-calling friends. “If this was 50 years ago, when all we had was a landline and it was expensive, how would we all have coped?

“If someone had said to me at the start, you’re going to be in your flat for the next 12 weeks except for a short daily walk, I would have freaked out. But I enjoyed not being busy. And I really enjoyed that 20 minutes in the local woods, looking at the birds.”

Again, this is deprecation. Steadman knows her avians. She is an ambassador for the London Wildlife Trust and 18 years ago wrote a children’s book, Spider!, urging awe rather than fear in the face of creepy crawlies (she speaks proudly of Freddy’s easy way with a millipede).

Steadman has yet to make the same inroads in America as peers such as Brenda Blethyn (who took over for the film version of The Rise and Fall of Little Voice, for which Steadman won an Olivier) or Julie Walters, with whom she seems to share some sensibility (when Steadman first met Victoria Wood in an M&S food hall, she was so overcome she couldn’t stop sobbing).

For Corden, it’s because she is primarily driven by a desire to have fun. “Some actors become withered and disillusioned. She’s never lost the twinkle in her eye for being with other actors and putting on a show. That’s just amazing to be around; it’s infectious. Nobody moans if Alison’s there.”

“I can’t imagine anyone not getting on with her,” says Johns. “She’s so open and demonstrative.” Between takes, they would sing show tunes to each other, he chuckles; with John Cleese – her co-star on sitcom Hold the Sunset – she would pretend to be a cat.

“We’d miaow at one another,” says Steadman. Cleese testifies to this curious companionship by email: “I am better at Maine Coons, Burmese and Ragdolls. Otherwise she has me licked.”

Cleese isn’t keen on the Guardian. Yet for Steadman, it seems he will make an exception, so he can tell people about her “great technical skill, playful and affectionate nature … constant, cheerful unflappability and complete absence of ego”. Right back in 1986 on Clockwise, “our connection was effortless,” he writes. “I’m ruder and naughtier than she and I think I sometimes appal her, but she is quick to forgive.”

That Steadman gets along so well with so many suggests not simply niceness, but also an adroitness at adaptation. She will purr or sing to suit. “Well, John is mad in the best way,” she says. “Eccentric, unique, quite extraordinary. We get along so well.” Yet you’re so different. “You can take what you want and ignore what you don’t. We need people like him around just to gee things up a bit.”

Hold the Sunset, about a pair of neighbours whose later-life romance is hijacked by the reappearance of her grownup son, is not a project that – despite its senior themes – feels as revolutionary as, say, Life. “Forty years ago,” says Steadman, “you wouldn’t have had a drama about a 70-year-old who suddenly decides to confront her husband. That just wouldn’t have been accepted subject matter.”

Ditto Care, from a couple of years ago: a toothy look at the care home sector in which Sheridan Smith battles to safeguard her mother (Steadman), waylaid by a stroke. Making that upped Steadman’s scepticism of a world she had long been worried about. “I read a book recently which asked why you never find a double bed in a care home. It’s always assumed that once you’re at that stage, you’re never going to want to cuddle anybody. I’ll be 80 in a few years. I don’t think I’m going to change.”

Let’s hope not. Later today, she and Michael will have gin and watch Channel 4 News and phone a friend. Soon, she will make a Mrs Punch to go with that first puppet for Freddy. Maybe it will be slightly less scary, perhaps she’ll even be able to show it to him in person. She has been round, she says, to the garden, at a distance. Freddy was disconcerted, asked if the virus had gone now. There was some confusion over a cuddle. “I gelled my hands and said to his mum, ‘Is it ok if I stroke his hair?’”

• 23 Walks is released on 25 September. Life begins on BBC One at 9pm on 29 September.

• The article was amended on 24 September 2020. The subheading originally mentioned Alison Steadman’s 40-year-career, rather than 50, and a picture caption misspelt Peter Davison’s surname. These have been corrected.