Critics often focus on Cuarón’s prowess with the long take. Certainly, his 12½-minute opening for “Gravity” — shot by Lubezki — ranks among cinema’s more jaw-dropping feats. For Cuarón, however, the long take is only a means to an end. “The Olympics of the long take don’t interest me,” he says. “Roma” contains scenes that are full of quick cuts. “It’s about how to lead to that thematic content through the filmic experience,” he told me, “so that it’s given in the filmic experience, not explained.”

He broke into an acid smile. “Almost all commercial film is film where you can go, buy your popcorn, sit in the movie theater, start eating. The moment that they shut off the lights, you close your eyes. You keep eating popcorn. They turn on the lights. You open your eyes, and you haven’t missed a thing. They told you everything. They’re like audiobooks, like illustrated radio novels.” Try that routine with any of the films that Cuarón wrote after “Sólo” and you’ll miss at least half the story.

After “Mamá,” Cuarón married the Italian actress and journalist Annalisa Bugliani, fathered two children with her (Tess and Olmo) and improved his leverage in Hollywood, which invited him to direct the third installment of the lucrative Harry Potter franchise. Even as he enjoyed success, however, many of his old insecurities remained. His father briefly resurfaced and tried to rewrite their personal history. (Never bothering, however, to learn the name of Cuarón’s oldest child, Jonás. “He called him Jason,” Cuarón says.) His prescient dystopian thriller, “Children of Men” (2006), which was nominated for three Academy Awards, disappointed at the box office. His marriage with Bugliani fell apart in 2008. As Cuarón wrote “Gravity,” he worried not only about supporting his kids, but also about paying divorce lawyers. Its story of a woman floating alone in space trying to survive catastrophe and grief may be read as an expression of his own emotional state.

Once upon a time, just seeing the front pages of The Hollywood Reporter or Variety in the elevators of the Chateau Marmont hotel could trigger an attack of self-doubt in Cuarón. Why had he turned down the chance to direct the film that now raked in those huge box-office receipts? Why had he passed on the script that now attracted such a star-studded cast? Shedding some of this insecurity is part of what Cuarón means when he said that he needed to develop certain “emotional tools” before he could make “Roma.” “This was the moment that I could do that story and make it stripping away all my creative controls. Let myself go, you know? The confidence to fail. Not to be afraid about what if it doesn’t work: Well, it didn’t work, and I didn’t hurt anyone. I’ll go back to make another ‘Gravity.’ There won’t be a problem. Nobody’s going to care that I went to be indulgent with my movie in Mexico, you know?”

For the first time in his career, he wrote a feature-length script entirely by himself. He wrote scenes randomly at first, using memory as a kind of Geiger counter to locate where the most potent material lay. He didn’t question whether the script was too long or too short, whether it had a first, second and third act, whether it would be boring. For a while, he thought the screenplay might not have any plot, but he still decided not to share it with his usual sounding boards — del Toro; Carlos; the director Alejandro González Iñárritu, whom he met in the 1990s. Lubezki wanted to shoot it — “I thought that it was the most beautiful script, probably, I’ve ever read in my life” — but shortly after preproduction began he had to depart for a family situation. “I would have never abandoned Alfonso in his most personal movie for anything else.”

Cuarón gave producers dates to research, having them look up, say, which television programs played on a certain night in 1970. I was particularly stunned by his recreation of the Corpus Christi massacre. Cuarón’s crew not only watched archival footage of the event, they also located and interviewed survivors. More than 800 extras were cast for the scene: students, paramilitaries, police, bystanders. These moments in “Roma” amount to an indictment of Mexico’s federal government, a dossier on a crime against humanity.

When I told Cuarón how struck I was by all of these historical details, he pulled out his iPhone. A few taps later, he showed me a 1971 photograph of several men squatting by beside a car, guns in hand, ready to fire at the students. ¡Ay! I exclaimed. They looked just like the actors in “Roma.” He smiled with satisfaction. “The same faces, the same wardrobe,” he said. “And it was about finding the exact face.” He pointed at the cars parked near the plainclothes paramilitaries. “In fact, it’s the same car that’s parked here and the other car that’s a little forward. The only thing is that because of the geographic position of where we were, it was reversed. Instead of looking there,” he pointed to the left, “they’re looking toward the right. In fact, I felt really stupid, because when we were doing the image correction” — in postproduction — “I showed it to my corrector, Steve Scott, and he said, ‘The only thing that you missed there was putting in the photographer.’ It sent me into a rage. I said: ‘Arrgh! How stupid! How stupid that I didn’t do that!’ ”