When I ask Wallace Shawn how he cast his latest stage work, What We Did Before Our Moth Days, the actor and playwright smiles matter-of-factly: “Well, I think that’s secret. I don’t think I’ll tell you.” It’s polite, to the point and sets a clear boundary: something that I soon discover that the charming 82-year-old is entirely comfortable with.

On an overcast Wednesday, we are in a restaurant atop the hip Manhattan arthouse cinema Metrograph, watching people trickle in a few days before a retrospective of his films opens there. Spending time with Shawn feels like stepping into his own constant sense of wonderment: something midway between a knowing shrug and puzzlement over his immediate situation. When the cinema’s publicist offers him a Twix bar, he cocks his head and asks what that is, but politely accepts one. (When she returns with more options, he opts for popcorn instead.)

Born in New York and a theater mainstay since the late 60s, Shawn has reached the farthest from his native island through memorable turns in Hollywood hits like The Princess Bride and Marriage Story. After making his first big-screen appearance in Woody Allen’s 1979 film Manhattan, he popped up in cult hits like Bob Fosse’s All That Jazz and Alan J Pakula’s Starting Over before indelible leading turns in My Dinner with Andre or Vanya on 42nd Street, both co-written with Shawn’s longtime collaborator André Gregory. Acting is a miracle, he tells me, because while actors “look like us, like we could do what they do … we can’t, really”.

It’s a curious comment from someone with more than 200 screen credits, but Shawn has been pushing himself to the limit this spring. On the two nights a week that Moth Days is not in performance, he has been restaging his blistering 1990 monologue, The Fever.

I saw Shawn perform that solo two nights before we meet. He entered to valedictorian applause, asked for sympathy should he need to consult the script he’d brought on-stage, and good-naturedly acknowledged the play’s length: “Admittedly, two hours is a long time, but it won’t be longer than that,” he pleaded. He didn’t end up needing the script, and its indictment of capitalism and moral decay poured out of him like lava right up till the two-hour mark. “That’s a physical feat which is at the absolute limit of my ability,” he says. “I am surprised that I can go through it without falling apart.”

But outside his collaborations with Gregory, Shawn feels disappointment that his ability to excel at avuncular, comic relief types in Clueless or Gossip Girl has not led to more dynamic parts. “I don’t think the returns are in,” he tells me. “Obviously, a lot of people don’t think I can act because otherwise they’d give me different parts. I’m clearly not highly respected as an actor by a lot of people, even if I’ve been sitting here for many decades, totally available. When I see myself, let’s say, in Young Sheldon” – the seven-season series where he held a recurring role – “I think: they cast me in that part and I acquitted myself. I don’t think they made a terrible mistake.”

Film fans agree that Shawn is a treasure. Earlier this afternoon I tagged along with him to sign film posters at the nearby Posteritati gallery, where the entire staff – including those not on-shift that day – had come in to see him. When informed of this, Shawn batted the praise away with his hand. He was thrilled to see an original British one-sheet for My Dinner with Andre, then asked if signing a postcard for Toy Story 2, in which he voiced a toy dinosaur, would decrease its value. When they heaped praise on to Jonathan Demme’s A Master Builder (2013), which Shawn adapted from Ibsen’s classic and starred in, he noted its dismal box office.

Shawn’s own financial situation was more or less solved in his late 30s. “I began to understand that I could make money by being a funny actor,” he says. “I was quite delighted to find that, even though my [stage] writing was not appealing to a large number of people, my acting apparently could be.” This realization spared him from having to water down his writing, leading to 17 decidedly singular stage works to date, including translations of Machiavalli’s bawdy comedy The Mandrake and The Threepenny Opera, his sole Broadway credit.

Moth Days, his latest, is a sort of primal return – whether he knows it or not. Structured as a series of direct audience addresses, it features four intellectuals (it could be titled The Man, The Son, Dad’s Wife and His Lover) detailing the ways a father’s affair influenced their lives up until their dying “moth day”. Performed by Hope Davis, Maria Dizzia, John Early and Josh Hamilton, the play is a poignant yet darkly satirical study of one upper-crust family’s attempt to come to terms with their own grief. Shawn’s own father, William, was editor-in-chief of the New Yorker for over three decades and, for about as long, had a semi-public affair with one of its writers, Lillian Ross.

Shawn says he never picks up the pen knowing where it’ll take him, and as to his Moth Days inspiration, points to the “absolutely true and well-documented” facts of his life. I ask if he’d heard Barbra Streisand’s reason for avoiding therapy (“I’m not that interested in myself”), given the two were born a year apart. He laughs and says no. “I think she doesn’t care, perhaps, why she behaves the way she does. It’s never gotten in her way. I understand it.”

Shawn has preferred to leave his motivations unexplored. “It’s about being self-protective,” he says. “I think I avoided psychedelic drugs up until this point, and psychotherapy probably for the same reason.” This makes The Fever’s caustic, often hallucinatory wallop that much more surprising, in retrospect. Shawn describes the monologue as “a very frank assessment of the role of that class of Americans” – the bourgeois – “as predators preying on the planet”, where an unnamed protagonist, shivering on a hotel bathroom floor, considers their oppressive role in the world while visiting a developing country. Its existential gallows humor is Kafka by way of Marx, prompting a nausea of, as Shawn puts it: “Uh-oh, my life that’s been very enjoyable has really come at the expense of people who have been crushed. If people in Sudan were prosperous, and if America hadn’t had slavery and if there hadn’t been a genocide of the Indigenous people, I wouldn’t be able to be doing what I’m doing.”

Shawn began performing The Fever in apartments in 1990 before staging it at New York’s Public Theater, where a New York Times reviewer savaged it as “a musty radical-chic stunt”. Does the artist mind the critics? “I’m afraid I read reviews, because I grew up in a print-loving household and I hate the idea that people are saying things about me behind my back.” He kept it pushing, and eventually HBO produced a film version starring Vanessa Redgrave in 2004. Being unliked, as well as earning the kind of respect he feels Arthur Miller was afforded by his 30s, was a motivator.

“I feel bad talking this way now because people have been extremely nice since I crossed the line into being 80, maybe even 70 or 75,” he says, almost embarrassed to acknowledge his writing career’s delayed acclaim. “But, before that … I always had higher expectations for myself.”

Shawn is attuned to the impression he makes; every word out of his mouth is as considered as a final written draft. Sensing I have more to discuss as he excuses himself for an appointment, we make plans to speak the following day. (Refreshingly, it doesn’t seem he employs a publicist.) Over the phone, I veer our conversation from the battle between hunger for esteem and artistic certainty into how that might reflect itself in his politics.

Shawn designed The Fever’s time and place to be intentionally “vague and abstract”, but its rebel militias and warm climates evoke the mid-century destabilization of Central America. Wallace, who often visited the region, says it was “written at a time when the United States totally dominated El Salvador [and] indirectly dominated Guatemala and Honduras”. Its lack of specificity is helpful for the play’s continued relevance (though less so for the state of humanity), and feels like a contemporary wake-up call from Shawn, who is also a longtime member of the leftwing, anti-Zionist organization Jewish Voice for Peace. “In my social behavior, I tend to be quite bland and agreeable, but there have been times …” he trails off, hinting at moments where he hasn’t shied away from confrontation.

He was shocked when Columbia University, where his partner, Deborah Eisenberg, used to teach, penalized its students for peaceful protests against the school’s ties to Israel. “Instead of defending these students, these academic institutions have given in to their donors and into the evil administration of Trump,” Shawn says.

“There have been some consequences to my becoming politically aware,” he adds. After the events of 7 October 2023, actors including Susan Sarandon and Melissa Barrera have described being blacklisted by Hollywood for pro-Palestine views. Has Shawn’s open support of Palestine cost him opportunities? He demurs, saying: “I don’t know any people who are enthusiastic supporters of the genocide in Gaza. I know a couple who would rather not think about it, but I don’t hang out with people who would be defending that.

“Undoubtedly,” he notes, “there are criticisms of me that I, myself, haven’t read.” Shawn is set on his beliefs and his work regardless. Once Moth Days wraps, it’s back to his particular duality: a voice role in Toy Story 5 then portraying the visionary 60s architect Buckminster Fuller in the forthcoming drama The Man Who Changed the World. The few around him with whom he doesn’t see eye-to-eye, he quips, “either they secretly know I’m right, or they like me personally and they’re capable of making the adjustments that ‘Wally is odd and that’s how he is’”.

• What We Did Before Our Moth Days and The Fever are at Greenwich House Theater, New York, until 24 May