In his most manic comedies, the great “reactor,” the unflappable Alan Arkin, looks like he’s on the verge of cracking up and blowing the take — scene after scene. He can’t wholly hide how tickled he is at what’s going on around him. It’s in his eyes, the barely-controlled grin that’s trying to bust out on his face.

You see it, here and there, in *The Russians are Coming, The Russians are Coming.” And there’s a moment in “The In-Laws” in which he glances towards the camera in his harrowing ride, clinging to the roof of a taxi he’s just clambered aboard, as if the actor playing the part sees the cameraman laughing at how this looks and wishes he could join him.

Legend has it that Arkin was so broken up by the great character player James Hong‘s improvised chattering Mandarin monologue, adding a magazine to his improv, that he’d run Hong off the set for his close-ups, lest the man break him up even when the camera isn’t on him.

I guess that’s why Arkin never hosted “Saturday Night Live.” They’ve had enough problems with “breaking” in the middle of funny bits over the decades.

Arkin’s “In-Laws” co-star Peter Falk, on the other hand, is Chris Freaking Walken in this movie — a cold-blooded comic assassin surrounded by mortals unable to keep a straight face as he demands “More COWbell.”

Their unlikely pairing paid dividends in this beloved farce from the late ’70s, a movie built on the wit of former Mel Brooks collaborator and future “Fletch,” “Soapdish” and “The Freshman” writer Andrew Bergman, director Arthur Hiller’s eye for a great sight gag and editing for antic energy, and stars and co-stars who knew where the laughs were and delivered them.

Check any list of the “Funniest Films Ever” and “The In-Laws” is on there. If anything, it’s grown in stature over the decades, a legend that was always funnier than “Some Like It Hot,” although perhaps a little shy of the mania of Billy Wilder’s funniest film, “One, Two, Three,” as hilarious as the Best of Peter Sellers or Mel Brooks or “early” Woody Allen.

Arkin plays a New York dentist named Sheldon Kornpett whose daughter (Penny Peyser) wants to marry this nice boy, Tommy Riccardo (Michael Lembeck). Yeah, even the surnames are amusing.

But on meeting the father of the groom, Vince Riccardo (Falk), Sheldon finds himself ensnared in an ever-enlarging fiasco involving stolen printing plates for a high-dollar denomination bill, a Latin American plot to print these and wreck the American economy and the machinations of the CIA, which Vince insists is his employer.

Sheldon is implicated, lured out of his dental practice mid-patient, chased, shot at, taken hostage and taken for a ride by Vince, who seems crazy, inept and yet insanely confident that “It’ll all work out” and that they’ll somehow survive, succeed and make their kids’ deadline-approaching nuptials.

“You were involved in the Bay of Pigs?”

“Involved? That was my idea!”

Their children and wives (Nancy Dussault and Arlene Golonka) are in the dark. The CIA (Ed Begley Jr.) feigns ignorance. The heavies chasing them — thugs, thieves, US Treasury agents, maybe even the CIA — include the most menacing Paul Smith, destined to play Bluto to Robin Williams’ “Popeye” the Sailor.

The New York shenanigans get loud and out of hand as Sheldon melts down, calms down and melts down again as every time he think he’s out, some fresh horror reminds him he’s not.

“Please, God, don’t let me die on West 31st Street!”

Hint — he doesn’t. I mean, they’ve got a date with a Central American firing squad in the third act, after all.

The mayhem begins at a jog, bursts into a sprint and tumbles, head-over-heels into hilarious scene after hilarious scene, with dopey lines fans can quote from memory.

The gunplay is all fun and games until they find themselves delivering the plates to General Garcia (Richard Libertini), dictator of tiny Tijuara, down Central America way. Avoiding assasins will require running “Serpentine, Shelly! Serpentine!” Because Garcia’s killers are not to be discounted.

“These are the best security men in the world. The used to work for J.C. Penney in Detroit.“

But the General, a goofy madman overly fond of his Señor Wences hand-puppet routine, runs a cut-rate firing squad. A last cigarette, but no lighter? No BLINDfolds?

“We have no blindfolds, señor, we are a poor country!”

The picture started life as a sequel to Hiller’s “Freebie and the Bean,” the first comedy to prove James Caan could be funny. But you can’t top the chemistry of Arkin and Falk, a polished double-act with Arkin amping up his reactor shtick (“The Russians Are Coming, the Russians are Coming”) an octave or two, and Falk worrying his Columbo-as-“The Cheap Detective” routine right into our funnybones.

It’s a movie that looks like every shootout and big action beat was filmed at 8 a.m., whose plot seems so shambolic it feels invented, on the fly, on set. The cultural references are as dated as the film stock. Just how fine a line this farce walked became obvious when Hollywood tried to remake it. No Falk. No Arkin. No dice.

Every scene sets up the next, every escalation works and every supporting player adds to the madcap complexity and comic inevitability of it all.

Every time you rewatch it, what you’ve forgotten tickles you again, what you’ve missed gives you a new grin.

Is that character-actor-in-the-making David Paymer as a New York cabbie Vince enlists in their crusade? It was the “Get Shorty” punchline’s first film role.

With the passing of time, the richness of the laughs — simple or exaggerated gestures, rising voices, underreactions and over-reactions — becomes the ultimate compliment for a classic.

It’s hilarious movie comfort food, worth watching again and again.

Some years back, I was grabbing a coffee with my then-brother-in-law at a Brooklyn cafe, when Luis leaned across the table and whispered “The In-Laws,” darting his eyes across the dining room at the one, the only, Richard Libertini.

It took me a minute to remember his name, but indeed, there he was — General Garcia in the flesh. We both glanced back to re-confirm, he caught our eyes and twinkled. We smiled and nodded, as New Yorkers or those impersonating them do. Anything more demonstrative would be uncool.

Luis and I swapped lines from the movie, quietly, giggling just as quietly.

But as he passed our table on leaving, I asked the Great Libertini, “Still no blindfolds?” He looks at Luis, whom he knew had “made” him first, leaned down conspiratorily and hissed “SERPENTINE!”

For actors, classic is any movie you’d love to be remembered for.

Rating: PG

Cast: Peter Falk, Alan Arkin, Nancy Dussault, Penny Peyser, Arlene Golonka, Michael Lembeck, Paul Smith, Ed Begley Jr., Richard Libertini and James Hong

Credits: Directed by Arthur Hiller, scripted by Andrew Bergman. A Warner Brothers release on Amazon, Movies!, Youtube, etc.

Running time: 1:43