Mahershala Ali broke down in tears on stage at the BFI London Film Festival on Saturday as he recalled the young boy at the heart of “Moonlight.”

The Barry Jenkins drama won him one of his two Oscars and recently marked its 10th anniversary.

During a Screen Talk hosted by Amon Warmann, Ali was discussing the dinner-table scene in which young Chiron asks Juan, the drug dealer Ali plays, whether he sells drugs. “You see this like crisis for this young boy who’s already in crisis,” he said. “It’s really heartbreaking.”

When Warmann asked what accounted for the film’s enduring power, Ali paused, overcome with emotion and wiping away tears. “Sorry, just need a second,” he said.

“Any time people put flesh on the bones of those who haven’t been seen or had their moment, I think it’s important,” he said once he had composed himself. “I think ‘Moonlight’ was able to make those characters people… make it personal.”

To play Juan, Ali drew on a man he had known all his life, an uncle-like figure he had grown up playing basketball with who also worked as a substitute teacher. The man was arrested and sent to prison while Ali was in college, which left the actor stunned.

“I thought of him a lot in the making of the film because of how human everyone was in my environment and in my upbringing,” Ali said.

He was candid about the backlash that followed “Green Book,” in which he played pianist Don Shirley opposite Viggo Mortensen. It was his first role of that magnitude, and the backlash included the controversy over what Shirley’s family had and hadn’t sanctioned.

“I still stand by it. I stand by the work,” Ali said, adding that he also stands by “the experience as a whole, and what it taught me.”

Ali, who won his second Oscar for the role, described going from feeling like a golden boy to wondering what he had done wrong. “I think I’m just sensitive in general, you know what I mean? So it definitely like had its impact, like for sure,” he said, adding that he was grateful for the exposure. “Like swords are folded in the fire, you know?”

Of his back-to-back Oscar wins, Ali said: “I felt a sense of accomplishment, which is not fulfillment.” He keeps the statuettes in a cabinet, out of his sight line, so he could get back to work.

Ali voices Aaron Davis in the animated “Spider-Verse” films, including a version of the character from another universe who appears at the end of “Across the Spider-Verse” and returns in “Beyond the Spider-Verse.” Asked how he tells the two apart, he said: “I don’t want to give anything away or spoil anything, I’ve got to be careful.” Pressed on whether he had finished recording, he joked: “I might not even be in it, man! I don’t know, man! I don’t know!”

He was more forthcoming about the process. Actors are filmed in the booth, and their improvised moments often make it into the finished film, sometimes prompting rewrites and extra recording sessions. “I think that’s why, honestly, the animation films will hold up better than live-action films,” Ali said.

The actor went straight from “Green Book” into “True Detective,” with only a week or two off, to play detective Wayne Hays across three timelines. Playing the older Hays meant 80-hour weeks, with four hours in the makeup chair from 3:30 a.m. The shoot ran seven months, and his daughter was a year old at the time.

“I just think about how I almost went crazy doing that part,” he said.

Two of Ali’s films are screening at the festival. He voices bandit leader Brendan in the animated “Wildwood.” In action thriller “Your Mother, Your Mother, Your Mother,” he plays Latif, a devout Muslim father and contract killer, and had three or four days to learn the fight choreography.

“It’s really more like dance than what people realize,” he said.

On playing a Black Muslim father, Ali said: “I don’t feel like Black fathers have the dimensionality that I would love for them to have.”

Now at work on the HBO series “Task” with Mark Ruffalo, Ali said he feels “exhausted for the right reasons” and “tired in the right way.”

“So that’s why I feel like I’m just getting started,” he said.