Lesley Manville was at the bus stop the other day when the comedian Simon Amstell spotted her and came up for a chat. He wanted to know what she was doing there. Manville affects bewilderment. “I said: ‘Well, why? I’m going to get the bus.’ He said: ‘I don’t imagine you getting the bus.’” He could see her on a bus, but not actually waiting for it, perhaps because she seems both grounded and grand. “I said: ‘I love a bus. I don’t want my life to be about taking taxis.’”

But buses are becoming trickier for Manville. It’s not just the “oomska oomska oomska” of tinny music emitted by other people’s headphones, which “irritates the fuck out of her” and is turning the public space into a private entertainment zone and spoiling the opportunity for earwigging. She can also hear her fellow passengers whispering: “It’s her off Mum! It’s her off Mum!”

The BBC Two sitcom was nominated in four categories at the TV Baftas on Sunday, one of which was Manville for female performance in a comedy, while Mum is about to return for its third and final series. No wonder Manville’s quietly devastating performance as Cathy, a recently widowed mother of one who is falling in love with her late husband’s best friend, is making it hard for her to pass unnoticed. Many bus rides are now spent clocking the furtive glances and wondering whether she will have to get off before her stop.

“I’m clinging on to it,” she says – the “it” being the bus, but also the anonymity she has avidly protected during a 47-year career across stage, TV and film that has won her a reputation not as a glittering national treasure, but as “a stalwart”.

The days of this reputation are numbered. Last year, she was nominated for an Oscar for her potently austere portrayal of Cyril Woodcock, the sister of Daniel Day-Lewis’s Reynolds in Paul Thomas Anderson’s Phantom Thread. We meet in the ballroom of the Langham hotel in London as she is preparing to fly to Canada to shoot Let Him Go with Kevin Costner and Diane Lane. Hollywood has discovered her, while the acclaim for Mum and Phantom Thread, on the heels of an Olivier award for Henrik Ibsen’s Ghosts in 2014, have turned her into a sort of poster girl for the older female actor.

Manville (far left) in Mum with Lisa McGrillis, Sam Swainsbury and Dorothy Atkinson. Photograph: Mark Johnson/BBC/Big Talk Productions

This is “a golden period”, she says, in which “film companies and television companies see there is a market” for strong female characters beyond the age of 40. “I’d just like there to be more of it, so that my girlfriends and peers and colleagues who are my age but not enjoying the kind of halcyon days I’m enjoying can share in it, too. More,” she says. “More, more, more. It’s important for young people to see that older people have not turned off a button and become this kind of muted, non-feeling entity.” She is speaking quickly now, in long sentences that showcase the kind of stamina for which she is known. “I do want to go out dancing and get sweaty and drink too much and go home at three in the morning, and I do want to have sex, and I do want to dress how I want to dress, and I want to have a life and be able to choose what I want to do; and I am over 60.”

This is something she discusses with her sister, Diana, who is four years her senior (Manville is the youngest of three; her eldest sister died in January). Diana says Manville is lucky to have a job where she is always meeting people of different ages, and that Manville’s world is “a bit different” from most. When they have this conversation in person, Manville likes to disagree. “But actually she’s right, it is different,” she says. “Because I have proper, serious, close best friends who are in their 20s and 30s. And I know they’re not thinking: ‘I don’t want to hang out with Lesley ’cause she’s over 60.’”

Those best friends include the cast of Mum, whom Manville says are in her life for ever. She has recently been to Champneys for a spa weekend with Lisa McGrillis and Dorothy Atkinson (who play Kelly and Pauline in the show). Sometimes they go shopping in Topshop, or out to eat.

I’m very single-minded, but I hope not in an arrogant way. I’m just incredibly self-sufficient

McGrillis had just had a baby when the second series was filmed. The commute was arduous, so McGrillis, her baby and her parents (who looked after the newborn) all moved in with Manville in west London for a while. Manville and McGrillis must have felt they were going home each night to a different sort of sitcom. For the TV Baftas, Manville, McGrillis and Atkinson were planning to get ready together at Manville’s house. This is not normal. “You’re lucky if every few years you pick up a nice person who ends up in your phone book,” she says. But, with the cast of Mum, Manville says: “I happen to actually, properly love them all.”

In an ivory crepe trouser suit, applying a layer of hand cream as she talks, Manville looks a long way from Mum’s Cathy. For a start, Cathy’s penchant – although that suggests a predisposition that overstates her interest in clothes – for “terrible jeans” makes her bum “look as if it goes on for ever”, says Manville, her voice rising in indignation. She is always asking her fellow cast members: “Does my arse look big in this?”

But Cathy and Manville have an interesting relationship. Manville spends a lot of time with her; although there were times when Manville used to think: “Come on, Cathy, come on! Lose it!” their relationship seems companionable as well as professional. There are leaves that Manville would like to take out of Cathy’s book: Manville finds it hard to break up with boyfriends, or turn down friends’ offers of work.

She has an extraordinary work ethic. “Exceptional stamina,” she says. While performing last year as a morphine addict opposite Jeremy Irons in Long Day’s Journey Into Night, she spent her days filming the TV show Harlots, removing her 18th-century makeup in the car each night between the TV set and the theatre. She is perennially busy; one job is never enough. I wonder if it comes from her working-class upbringing. Her father was variously a plumber, a printer, a cab driver and a bookmaker. She grew up in a council house in Hove, East Sussex, and then a council flat.

Manville with Peter Wight in Mike Leigh’s film Another Year.

But Manville thinks not. Her upbringing “wasn’t grim. We were never poor.” As well as the sundry jobs, her father also gambled (a big loss led to him declaring bankruptcy when Manville was four, hence the council house), so they were irregularly wealthy. “He wasn’t a miserable loser.”

For a few years, Manville owned a pony. On Sundays, they lunched out. Her father was a crooner. He liked to duet with her eldest sister on Frank and Nancy Sinatra songs; the family holidayed at Butlin’s in Brighton and loved the open mic nights.

The experience that really informed her work ethic, she says, was leaving her secondary modern aged 15, before her O-levels, to enrol at the Italia Conti stage school in London. Her parents mounted no defence of a general education. They stepped back and let her get on with it, which fostered a sort of single-minded self-parenting. By 17, Manville lived away full-time, while presenting a Blue Peter-style show in Plymouth, Devon.

“I was all on my own, living in a rather seedy hotel with lots of rather seedy businessmen, going out for dinner on my own every night,” she says. I can’t help feeling sorry for the teenage Manville, who sounds pretty lonely. But the 63-year-old version insists: “I wasn’t crying into my pillow every night.”

I wonder if hindsight has helped her to understand this period as formative and therefore to feel more protective of the value of the experience than of her younger self, because she says: “There were no mobile phones. I was very much on my own. So I just rolled up my sleeves and got on with it.” She has a knack for asserting the need for no fuss while emphasising the ordeal.

“I still roll up my sleeves and get on with it. I’m never late. I always know what I’m doing. I don’t let people down on set. I’m very, very professional. I’m very pleasant,” she says, with a briskness that makes pleasantness sound highly professional. “I’m not arsey,” she adds. “You know, I don’t big it up or anything.”

“Getting on with it” is something of a mantra for Manville. It is so multipurpose, and used with such defiance, that I can’t help wondering if at some level she feels she is still defying expectations. She had “a quiet chip” on her shoulder in her early days at the Royal Court, as a person “literally without an O-level” working alongside intellectuals such as Max Stafford-Clark and Caryl Churchill.

She applies the same “getting on with it” to her experience as a single mother. Her first husband, Gary Oldman, left when their son, Alfie, was three months old. Although she says: “We’re friends. We like each other. It’s nothing horrible,” it is also true that early parenthood brought terrific challenges. She breastfed backstage, and stuck to theatre so she could care for baby Alfie in the day, then go off to do Miss Julie or Top Girls, before returning home to get up for him in the night. (She, Oldman and their son recently celebrated both actors’ nominations together at the Oscars.)

“Getting on with it” is also, apparently, a favourite phrase of Mike Leigh; Manville has worked on 11 of his films, more than any other actor. “We have said time and again in our lives, in our friendship, in our working thing: ‘Just get on with it,’” she says.

Manville with her first husband, Gary Oldman, in 1986. Photograph: Alan Davidson/Rex/Shutterstock

She thinks it is a pity that actors who try to circumvent the natural ageing process with Botox don’t take the same advice. “Chrissake, get on and deal with it!” she says. “We’re going to die, we’re going to get older … I just don’t think some of them realise how silly it is. Not just how they look – you know, those foreheads that don’t move. But the whole notion of it. Being 80 and looking 40. Highly intelligent women. But I understand the fragility.”

It has never been suggested to Manville that she might consider Botox, and she wouldn’t countenance it. She hopes that her “journey with Hollywood” carries on being a journey and that the likes of Paul Thomas Anderson are “not going to want some Botoxed ninny”.

Manville has extraordinary self-possession, the sort that conveys abundant self-esteem and humility. I suspect this is a thread that runs through her work, back to the early days in that seedy Plymouth hotel, when the job was anodyne but the situation demanded she get to grips with who she was. As she puts it: “Although I didn’t know what kind of an actor I could be, Lesley was forming.”

Manville refers to herself in the third person on several occasions, but it never sounds egotistical – more that she is a person in regular communication with herself. She once said that single parenthood provokes introspection, but she sounds very sociable to me.

“I am. But I spend a lot of time on my own and I need it,” she says. “I am very inward-looking. I listen to my own voice and instincts. I’m very single-minded, but I hope not in an arrogant way. I’m just incredibly self-sufficient.”

Older people often have certain things they like to do at set times, she says, and this is how her preferences have hardened. She dislikes it when people send a car for her. “I really want to drive myself to work. I like to start the day in my own space.” She says taxis “don’t sit well” – another favourite phrase – while fidgeting in her seat, as if the very thought makes it difficult to get comfortable.

She likes to make her own lunch, apply her own makeup and answer her own emails. She doesn’t have a personal assistant, staff being another thing that doesn’t sit right. “I’ve got nobody at home looking after me,” she says, in a voice that seems to invite a morsel of pity without a yearning for change. If a driver is absolutely necessary, she tells them straight away: “Hello, lovely to meet you. We’re going to blah-de-blah. Please don’t drive too fast, ’cause I don’t like it.”

Eschewing some of the perks of the job is about more than wanting an ordinary life, though. For Manville, it is a professional choice. There are disadvantages in the “rarefied” life. From earwigging on the bus (“heaven”), to singing Somewhere Over the Rainbow by way of thanks to the grocery boy when she was a kid, she has had a life gilded by the joy of unvarnished experiences. “Because of the life I had, and the life I continue to have, I understand people quite well – the essence of people – and then convey that,” she says. “So I think I have a humanity.”

Series three of Mum begins on 15 May on BBC Two at 10pm