In a new Off Broadway play, Linda Lavin shines as a woman paid to say what an ailing young man cannot.Disease, dying and death are usually depicted as wretched or bloody onstage. We’re meant to cry or recoil.As you might guess from its title, though, “You Will Get Sick,” which opened on Sunday at the Laura Pels Theater, is more matter-of-fact. It seems to promise a bald memento mori in the form of a fortune cookie.Yet the play, written by Noah Diaz, directed by Sam Pinkleton and starring the evergreen Linda Lavin, is far more than that. Neither prosaic nor clinical, it defies all expectations for a story in which the main character receives a fatal diagnosis, telling the tale in the most lively, surreal and surprising ways imaginable.For one thing, Lavin, who is 85, does not play the character who’s ill. Turning the template upside down, she instead plays the caregiver, Callan — if you can call someone a caregiver whose every act of care is minutely monetized. Never lifting a finger without naming a price, she’s more like an end-of-life TaskRabbit, having answered an ad from a man seeking someone to listen to him admit that he’s sick.That he can’t actually say the word reflects on the way his life as a millennial — he’s in his 30s — has failed to prepare him to envision such a fate. But for his initial payment of $20, he purchases the opportunity to practice his confession by telling Callan that his limbs are growing numb as his illness progresses. Soon he will be paying her more to break the news to his narcissistic sister (Marinda Anderson) and others in his orbit. Even his co-workers don’t know why he hasn’t been at work.The playwright, making his New York debut, is withholding too. He elects not to name the character (he’s simply called #1 in the script) or even the disease, which resembles multiple sclerosis. But in Daniel K. Isaac’s typically and appealingly restrained performance, we understand much more. This is a man who protects himself against too much feeling by keeping the flow of information to a minimum. The flow of money replaces it.From left, Marinda Anderson, Isaac and Lavin. All the actors except Isaac play multiple roles.Sara Krulwich/The New York TimesSuch omissions and substitutions are part of the play’s overall approach. We never do hear #1 say the things he wants said; that service is provided instead by a disembodied narrator (Dario Ladani Sanchez) and of course by Callan, who, aside from the paid-for retellings, turns the story into a monologue for her night-school acting class.If this Cubist approach sounds too clever, it is in fact functional. The second-person narration (“Your hand goes numb,” says the voice) reproduces in the audience the sensation of dissociation #1 feels as his body starts to operate independently of his will. And the third-person monologue (“His balance isn’t right,” Callan declaims) demonstrates how our stories, even when buried, may yet leach into the world.For Diaz, theater is clearly part of that process; the slightly indulgent acting class sequences engage in some affectionate if too easy satire. (“There is no can’t,” says the teacher. “There is sometimes cannot … There is mostly maybe … We call that do.”) Yet when #1 accompanies Callan to a session one evening, the trite instruction to “live inside our bodies” becomes, in a quietly joyful moment between them, profound, experienced from opposing side of wellness.Lavin’s wit is in full bloom playing a woman who, unlike herself, is a terrible actor and a worse singer. (When prompted to walk like a lion, she’s suddenly Gwen Verdon doing Fosse.) Callan is as rich as any role she’s had in years — and even richer in some ways, because it doesn’t trade, as her characters in “Our Mother’s Brief Affair” and “The Lyons” did, on her innate glamour. Far from it: Her Callan is that woman you see on the subway, pawing through a dirty tote bag, her auburn perm three inches grown out.And yet, as a foul-mouthed, don’t-mess-with-me urban lady with sincere if hopeless dreams in her head, Lavin has never seemed more vital, sly and fearless. When she admits that she wants to play Dorothy in “The Wizard of Oz” — to which #1 incredulously responds, “Did this Dorothy see the trials of war and age 60 years?” — you somehow feel the deep sense of the ludicrous self-casting.Such underground connections are at the heart of “You Will Get Sick”; Diaz is working a surrealist vein that doesn’t mean to make an argument so much as to plant the seeds of one you can have with yourself later. That all the actors except Isaac play multiple roles — Nate Miller plays seven, marvelously — suggests layers of correspondence among them. Most of Miller’s are fearful, for instance, and Anderson’s are all hilariously tin-eared. When #1’s body starts turning into hay, you may begin to see that they are familiar archetypes as well.Anderson, above center, leads Isaac, Lavin and Nate Miller in animal exercises during an acting class.Sara Krulwich/The New York TimesThe hay — not to mention the marauding birds, “The Wizard of Oz” and the narrative legerdemain — could easily have made “You Will Get Sick” too self-consciously poetic, its spray of images dissipating too quickly. But Diaz, who is 29, has had time to refine and tighten the script since he wrote it in drama school in 2018. In any case, it flies by, feeling even shorter yet fuller than its 85 minutes, especially as the imagery coalesces in a neat pull of strings at the end.That a play in which sadness is always biting at your fingers comes off this light and funny in performance requires a great deal of discipline. Some of that clearly comes from Pinkleton, whose direction trusts the material deeply enough to ask the audience to come toward it instead of the other way around. No surprise that he is also a choreographer, nominated for a Tony Award for his work on “Natasha, Pierre & the Great Comet of 1812”; he’s alert to the play’s internal rhythms.The Roundabout Theater Company production is also alert technically, with a trick box of a set by the design collective dots, lighting by Cha See, costumes by Michael Krass and Alicia Austin, and — especially — a powerful sound design, both apocalyptic and psychological, by Lee Kinney. He helps you believe in the existence of the soul, and also the forces that threaten it.If this all makes “You Will Get Sick” sound avant-garde and difficult, that’s part of the problem Diaz is addressing. Disease, dying and death are the opposite of avant-garde; they’re old news. And they’re difficult only in the way old news is: They happen to other people, always in the past. When it comes to our own demise, we don’t want to talk about it. Perhaps that’s why, in “You Will Get Sick,” we gladly pay Diaz to do it for us.You Will Get SickThrough Dec. 11 at the Laura Pels Theater; roundabouttheatre.org. Running time: 1 hour 25 minutes. More