In 1986, Robin Williams was at a crossroads. “Mork & Mindy” was four years behind him, and his movie career had been foundering. His marriage to Valerie Velardi, his first wife and the mother of their son, Zak, was unraveling. But he still had two reliable sources of joy in his life: his live comedy act, and Zak.

Zak was just 3 at the time, but in his own way, he played a pivotal role in Robin’s stand-up tour from this period, which culminated in a performance at the Metropolitan Opera House:

Williams was more than a year away from his cinematic breakthrough in “Good Morning, Vietnam,” but he was in peak stand-up shape — clean and sober, elastic and energized.

Over the span of this hour-long set (which was also released as the Grammy Award-winning comedy album “A Night at the Met”), he plays his usual grab-bag’s worth of characters: Robin Leach, Ronald Reagan, a ballet-dancing Tom Landry and, in a whispery, falsetto voice, Zak.

Zak as a ceaseless source of existential queries that his father cannot always answer. Zak as a less-than-ideal beneficiary of a trip to Disneyland. Zak as the inheritor of Robin’s fondness for a certain obscene phrase:

These are just jokes. Some are based in the reality of Robin’s real-life experiences as the father of a small child, while others are humorous flights of fancy that could plausibly happen to a parent.

But then, in the waning moments of the show, Robin conjures up an exchange between himself and Zak that is essentially un-invent-able: It’s funny, but also achingly sincere and unexpectedly poignant. It’s a rare expression of openness from a performer who was more comfortable with his audience at arm’s length.

First, Robin is playing himself. If you only knew him from his TV role as the gentle alien Mork, you might be surprised by his intensity here.

There are times my son looks at me and gives me that look in the eyes, like, Well, what’s it gonna be?

In an instant, he has become Zak, looking upwards with wide-eyed earnestness.

These are the kinds of quick-change pirouettes Robin has been doing in his act for almost a decade now, but just wait — this one’s going to be a little different.

The camera cuts to a wide shot of Robin looking down where Zak would be. The framing tells you how large the father looms in the son’s perspective.

Robin, meanwhile, is doing something he rarely allowed himself to do in his act up to this point: He’s speaking as himself, in his natural voice, with no character or accent to hide behind, and his awkwardness is palpable.

An uncomfortable moment passes in silence — no more than a second or two, which is practically an eternity for a performer hailed as a relentless motormouth. As he slows down time and indulges in the stillness, Robin is communicating that, for once, something significant is at stake here: his relationship with his son.

Though none of his viewers could know this yet, Robin is all too aware that if his marriage fails, his connection to Zak could be jeopardized. But he steels himself, swallowing his uncertainty and putting on an air of confidence, as all parents must from time to time, and extends a hairy hand down to his son.

With his hands on his hips, Robin assumes a pose reminiscent of a proud pop at a Little League game. He’s setting up one of the few genuine, by-the-book jokes of this bit — a callback to one of his opening lines of the night.

He asks Zak: “Hey, how do you get to the Met?”

In Zak’s voice he gives the correct response: “Money.” When Robin cracked the joke at the start of the show, he was mocking himself and the incongruity of an uncouth comic performing at this august theater. Now, the punchline is something more touching — a sign of a true bond between father and son.

The soft-spoken gentleness that Robin would later use to greater effect in films like “Dead Poets Society” peeks through in this moment as he tries to reassure Zak. “Come on, pal,” he says. “You’re not afraid, are you?”

But — ha-ha — it was all just a head fake, as Zak punctures the solemnity by repeating his favorite obscene phrase of a few minutes earlier.

As Robin exits the stage with his hand held skyward, he’s miming the act of little Zak holding onto the hand of his father for support. But it also suggests Robin going off on a journey of his own: the start of a years-long process of learning to let down his guard and be more of himself onstage.

When this same sequence was played in tribute on the 2014 Emmy Awards, just two weeks after Robin’s death, it took on an eerie, otherworldly quality. It seemed to emphasize Robin’s solitude as he entrusted his hand to an unseen figure and stepped into darkness.