Succession star J Smith-Cameron: ‘There’s a bewildered vulnerability to middle age that is ripe to be explored’

When J Smith-Cameron tells me that she wishes we'd been able to meet in a bar rather than a rehearsal room near Waterloo station – we're on a sofa beside a galley kitchen, listening to the sighs of a cranky immersion heater – I'm caught between believing her, and assuming she's just playing the trouper. If it takes one to know one, she does indeed strike me as a martini kind of a woman; like Gerri Kellman, the character she played with such brilliance in the TV show Succession , I can just picture her biting decisively on an olive in the low-lit gloom of a Manhattan hotel. But this Thursday evening, she also looks tired, a bit fuzzy at her edges. It's 6.15pm, and she has been rehearsing Seán O'Casey's voluble play, Juno and the Paycock, all day. In her shoes – she's wearing trainers, with a black jumpsuit – I'd already be on my way to a hot bath, an episode of something schlocky and an early night.

It turns out, though, that the purplish shadows below her eyes have relatively little to do with the demands of her director, Matthew Warchus, or her co-star, Mark Rylance. Having arrived in London from New York three weeks ago, disaster struck almost immediately. "I don't know whether you're aware of this, but I had to have an emergency appendectomy," she announces, curling a lock of hair carefully around an ear (a gesture that is very Gerri). "And I'm still sort of getting my strength back." How horrible, and when she's so far from home. "Yes, it was a big thing, and my husband [the playwright and Oscar-winning screenwriter, Kenneth Lonergan] couldn't come because he's opening a play in New York. But I just feel very lucky. I want to do a whole series about St Mary's [hospital, Paddington]. They were wonderful. They were rock stars. I sent over a gift for them today because – oh my God – they were so skilful and hardworking, and their work ethic was so aggressive, you know?"

The trouble began with a pain in her abdomen: "I could barely straighten up." She left a message with the company, saying that she could not come in to work. Should they call a doctor? Yes, she said, perhaps that would be wise. The doctor suggested hospital – quickly. "They admitted me straight away, and put me on antibiotics. There were more urgent surgeries than mine, so then I had to wait a day. I came out on a Friday evening, and they [the company] didn't call me; they let me rest until the following Tuesday, and then I did half-days for a while, which was draining, but also good. This was, I guess, two weeks ago. Today is only my second full day back. Everyone has been extremely gracious and understanding and... I know, I know! I'd been looking forward to all this for weeks. I was so excited, and then all hell broke loose." How does she think it's going now? I hesitate to say it, but the first preview is in only three weeks' time. "Touch wood," she says. "Everything is fine, apart from me being a little behind." Never forget that she is a pro: long before Succession creator and showrunner Jesse Armstrong arrived with his one-percenter monsters, at which point she became famous in a whole new way, Smith-Cameron was a bona fide, Tony award-winning Broadway star.

In fact, this is the second time she has played Juno Boyle, longsuffering wife of the drunk and wastrel Captain Jack Boyle (off Broadway in 2014) – though this may be a hindrance as well as a help. "I loved it [the first time], and I sort of didn't get my fill of it, so when I heard that Matthew was thinking of doing it, I said: can I just throw my name in the hat? The play is 100 years old this year, which is crazy and cool, and without drawing any direct comparisons, I think it addresses some of what's going on in Gaza [O'Casey's tragedy is set in working-class Dublin against a backdrop of sectarian politics]. On the one hand, this part is [for me] like tea that has been steeping. I wasn't too fussed about the script before I got here. I thought: it's all in there somewhere; it's going to come back to me. But it's also disorientating, sometimes [having done it before], both familiar and yet completely different."

It is, she says, "a little daunting" to be making her London stage debut alongside a "titan" like Rylance. But just to be here is a thrill. "Our theatre descends very directly from Britain. This is where Shakespeare performed! We're playing in the Gielgud [on Shaftesbury Avenue] and a couple of weeks ago, we got to walk around there – and there are these incredible photographs of The Importance of Being Earnest." (Gielgud starred in Oscar Wilde's play in 1939 in a cast that included Edith Evans as Lady Bracknell and Margaret Rutherford as Miss Prism.) "Broadway has its own pedigree. But this city has this incredible history for anyone who loves theatre." The appendectomy has temporarily upended her plans to make like a local – she's ordering takeouts, not haunting the aisles of Sainsbury's – but once she gets her stamina back, she's going to drink it all in. "I don't know where to start. It's such a beautiful city. Everywhere you turn is like those classics you read as a child. This is where it happened: I mean all of Dickens!"

Smith-Cameron was born in Kentucky, and grew up in South Carolina (the J is for Jeannie, a name she felt was too "little-girlish" once she began attending auditions), and her voice, warm and confiding, still brings to mind the south at moments. The daughter of an architect, she got interested in acting, in a roundabout way, courtesy of her big sister, to whom she's still close. "She's my senior by almost 10 years, and she was going to be the actor in the family; I played the violin. And she did pursue it, but it's a hard thing to pursue in the south. Not only was there no professional theatre in my town; there wasn't one in the state." In the end, her sister became a teacher. "In high school, a friend dared me to audition for the school play, and under the cover of my sister no longer being around – I was nervous of being compared with her – I got the part. It was then very clear that the violin was not in my future."

She went to Florida State University, largely because it had a school of theatre. But after a year, she dropped out. "I was always doing plays, sometimes more than one at once, and I started getting summer theatre jobs, and I had an independent movie, too – and so, at a certain point, I just never came back." This made her parents anxious, even years later. "I remember my mother visiting me and Kenny in New York, and her saying to him: you know, I just really wanted her to get her degree, because you need something to fall back on. And he said: er, I think she's going to be OK. I'd been making my living then for decades!"

At the outset, though, she was careful to be realistic when it came to her career. "I still had a lot of fear. It just seemed like an impossible thing. I began to be aware of regional theatres: in Florida, there is a good regional theatre, and in Louisville, my birthplace, there was a repertory company. I thought: I could get that kind of job, and it took the onus off thinking of acting as all or nothing – which I think a lot of people do: either you're Meryl Streep, or you're a loser."

Arriving in New York, she was almost never out of work – her first job in the city was on Broadway – but even so, all of her courage had to be deployed in the move. "I felt... I don't know how to say this. I felt, like, sort of adventurous. Just the act of moving to New York was so bold and audacious. I arrived with a $200 unemployment claim [a benefit] and stayed on friends' sofas for a while. But by the end of the year, I had the Broadway job and I was earning a decent amount of money – and thank God, things just followed. I didn't ever have a long fallow period." She loves doing TV and movies, but theatre is her passion. "It's so actor-oriented. You're responsible collectively for telling the story, and your fellow man up there is depending on you. It's almost primeval: [for the audience] there's that feeling of shelter. Everyone packs in and quietens down, the house lights go down. It's group concentration: everyone thinking about the same thing."

Is it also the case that theatre provides more roles than screen for older women [Smith-Cameron is 67]? "Well, I think older women are enjoying a surge of breakout older parts. It used just to be matriarchs, these ball-busting boss characters. But there's a sort of bewildered vulnerability to middle and older age that is very rich and ripe to be explored, and people are beginning to do that." Does she watch cosmetic surgery-crazed women around her making a kind of Faustian pact in which they trade the ability to look expressive for looking a little younger? "I think it varies from performer to performer. People who become famous when they're very young and beautiful might feel lost as they get older. Those who do well in middle age aren't caught in that trap." She stops herself. "I don't mean to suggest that they're leftover unattractive people! Just that people who look real don't feel the same burden."

Some actors would be cross – insulted, even – to be asked constantly about a hit TV series on which they worked relatively late on in their career, as if everything they'd done until this point hardly counted. But Smith-Cameron can't get over her luck that the award-winning Succession came along. Gerri was written for a man, and even after she'd auditioned and bagged it, the character was only supposed to be in four episodes: "I wasn't in the pilot, but I willed myself into every episode of the season after that." The experience was marvellous: almost like improv. "We all created our parts, in a way. I thought Gerri should have glasses, that she should wear her hair up. And then there was the camera, moving round as if it was one of us. We would throw in improvisations. If you said something funny, it would just swing back to get the reaction. You had to assume you were always on camera."

Even her psycho-sexual relationship with Roman (Kieran Culkin) was a kind of accident. "We were in a bar, and we had this little exchange, and then I left – and apparently Kieran kind of looked back at me, and then just as he turned around, I looked back at him. The writers were giggling, and when we came back for the next season Alan Ruck [who played Roman's brother Connor] said: so, I hear you're Roman's love interest for season two. I was, like, what? I didn't believe him at all." What kind of parts came her way because of all this? "I started getting versions of Gerri, and they felt like weak tea, because there is only one Gerri, and I was an unlikely person to play her in the first place."

Culkin was already a friend, having appeared in Kenneth Lonergan's 2011 film Margaret (in which she also starred) as well as in one of his plays. So, I have to ask: how does one meet a leading American playwright and Oscar-winning screenwriter (he wrote and directed Manchester by the Sea and co-wrote Gangs of New York)? "I just got lucky, I suppose!" she says, with complete delight. The couple have been married since 2000, and have a daughter, Nellie. "I had been married before, and that was short-lived, and I was just at the point of wanting to get back into the world of dating." A friend asked her to take part in a run of short plays, one of which was by Lonergan (eventually it grew into his movie, You Can Count on Me ). She watched him acting – he had a role, too – and he was "adorable, so funny... and he has this quiet affect, and you really kind of watch him". Later on, she teased him on the stairs outside the theatre – he hadn't heard of the playwright William Inge even though, unlike her, "he'd been to college".

The final night of these plays arrived. "I thought: OK, I've got to try to flirt with the grumpy boy again. He had flowers for the cast, and I knew they weren't for me, but I said it anyway: oh, are those for me?" The back and forth continued. She asked him if he'd missed her yesterday, and he said: "Can you doubt it?" She goes on: "People started coming into the green room, and I was tingling." After the curtain, everyone adjourned to a bar. She was with Alison Janney, star of The West Wing, who wanted to go to a place called Nadine's – which Smith-Cameron refused to do, because she needed to wait for grumpy guy. Tell him to come to Nadine's! said Janney. Smith-Cameron said she was playing it cool. But Janney forced her – "Go get him!" – and she did, walking the length of the bar to what would turn out to be her future. It's like a romcom, I say. "It is," she replies. I swear she is flushed at the memory. She's missing him terribly, but he will be here for her opening night.

The play runs beyond the date of the US election, which will be strange for her (though she has a postal vote). She is a vocal Democrat, and though I don't wish to crimp her mood, I wonder how she's feeling about the result. "Cautious optimism," she says. And then: "Cautious, cautious optimism." She cannot understand what would motivate anyone to vote for Trump; to cling to him even after his performance in the debate with Kamala Harris. It amazes her that the whole thing may turn on a few hundred people in just one state. But for all her involvement when she's at home, there's very little she can do from here. The play is the thing, now – and when the room fills suddenly with the enticing smell of noodles and soy sauce, I understand that this is my cue to exit, stage left.