Steven Spielberg will celebrate his 50th year as a film and TV professional in 2019, but the Hollywood icon shows no signs of slowing down. At 71, his enthusiasm for his craft seems little changed from the 1970s and 80s when he made early-career classics like Jaws, Close Encounters of the Third Kind and ET.

The proof, when we meet at Claridge’s in London, is not only the passionate (if slightly junket-weary) way Spielberg speaks, but also the work itself. As if making The Post, the film he’s promoting, in just under nine months wasn’t impressive enough, he did it at the same time as working on another feature, Ready Player One, which will be released in March.

“I’ve never made a movie this fast in my life, but we didn’t sacrifice quality,” he insists. “If I had another 12 months it would have been the same movie, I really believe that.”

I believe it, too. Nothing about The Post suggests sacrifices were made for speed. It is a gripping re-telling of the events surrounding the Washington Post’s battle to publish leaked parts of a top secret government study that became known as The Pentagon Papers, which damningly revealed how successive presidents had lied about the progress of the Vietnam war and sent thousands of young men to fight, despite knowing that it was unwinnable.

The decision to publish or not was in the hands of the Washington Post’s publisher, Katharine Graham (played by Meryl Streep), daughter of the organ’s late Jewish owner, Eugene Meyer, who staked her freedom and the future of the newspaper against legal action from the Nixon White House.

Spielberg knew when he read the screenplay that this wasn’t a film that could wait, even if, as he told a reporter, the idea of committing to it while in the middle of Ready Player One was “only half sane”.

“I just thought this was the right time to talk about freedom of the press,” he tells me. “To talk about the right of free speech. Talk about, celebrate, the hard work of journalism and news reporting, and how good journalists hold themselves to print their own principles in order to tell the truth, and be able to back up that truth with facts.”

With Trump in the Oval Office and at war with swathes of the US media (and most recently the author Michael Wolff and his publisher), it didn’t take much imagination for Spielberg to see that the “pendulum [had] swung all the way from 1971 to 2017, and, with a slight correction of the 1 and the 7, see that we’re in the same situation today, only worse.”

Americans, he says, are “listening to the news with new ears . . . and seeing the truth being labelled fake if the truth doesn’t please those who are calling it fake.” Truth is becoming whatever anyone says it is; falsehoods are “alternative facts”. “I said, ‘This is the time to tell the story, when we can all become part of this national conversation.’”

I suggest to Spielberg that this toxic slipperiness is part of the reason for the troubling spread of Holocaust denial on social media, and ask him if the Shoah Foundation, which he founded in 1994, and which has collected thousands of video testimonies of Holocaust survivors and witnesses, is aware of this. “It is,” he confirms, adding that the far right demonstration in Charlottesville, last August, when “National Socialism and neo Nazis had their moment in the sunlight . . . was a horrible period of time. Not just for Jewish people, but for all people.”

He reveals that one of the reasons he made Schindler’s List in 1993 “was to give the deniers no ammunition to deny. And for a long time that was effective — the denying it [the Holocaust] stopped for many years,” claims Spielberg. “But, of course, like everything, it resurfaces, because toxicity tends to come from the ground and resurface. So at the Shoah Foundation we are vigilant and continuing to get the message out: never again.”

This is getting harder, he admits, but the foundation’s strength is an archive that it can disseminate to museums, universities and, importantly, schools. “Then when kids are learning about social justice and they’re learning about civics and they’re learning about social science, they suddenly learn about the Holocaust, and it becomes something that they can tell their kids about some day. You have to start with the young.”

This contrasts with the ignorance Spielberg faced growing up as part of the only Jewish family in a non-Jewish neighbourhood in Phoenix, Arizona. His sister Nancy told me for these pages how they were called “dirty Jews” by other residents Their sense of isolation is partly why her brother identified with Katharine Graham. In The Post, Spielberg makes her isolation in the male-dominated world of newspaper publishing feel almost visceral, and says this harks back to his childhood.

“This is a feminist film, a movie about a woman who was discriminated against because she was a woman, even though she was in control of the men who were discriminating against her . . . [and] I relate to that very, very much.”

Of course it was ethnicity rather than gender that set Spielberg apart from other kids, but the feelings of loneliness and separation are similar, he suggests. “I know what it feels like to stand out. I know what it feels like to be looked at differently, to be discriminated against. I know what it feels like to be excluded from conversations and playtime and sports because I kind of wore a sign. That was a difficult kind of thing to endure as a kid.”

Every day felt like a fight for survival: “Just getting home safe, for me, was an accomplishment,” he sighs. “Then I’d have to go back to school the next day. So, in a sense, I can relate to how Katharine Graham felt isolated and surrounded and beset and besieged.”

All Spielberg really wanted as a boy was “to be social, instead of hiding in my closet or hiding in my bedroom and closing my door. I wanted to be accepted. I wanted to be liked. And I wanted the people who weren’t liking me to like me.” The key, he discovered, was a camera. When he started making movies, everyone suddenly wanted to be his friend. “The camera was sort of my date,” he laughs, recalling how a nerd in the film Animal House gains respect by taking a beautiful girl to a frat-house party.

“My camera was the good-looking person on my arm and it made me a lot of friends, because everybody wanted to be in front of it. They all wanted to appear in the movies. So a lot of the kids that were against me suddenly wanted to befriend me so they could get in front of the camera.”

In some ways nothing has changed — everyone wants to work with Spielberg. The director isn’t blasé, however. He brought Meryl Streep and Tom Hanks (as the Washington Post’s editor, Ben Bradlee) together in a feature for the first time in The Post, and talks about it as though he’d won a golden ticket to Willy Wonka’s chocolate factory.

“I’ve done a lot in my career and I can still be amazed and I can still be completely surprised,” he says excitedly. “And I’m glad I’m not jaded yet because every single day I was just saying, ‘I am the luckiest director on the planet Earth to have those two icons in front of MY camera. That’s MY camera. This is MY movie. And they’re in it!’ I was proud of that.”

This is the kind of attitude that keeps him looking forwards and his film-making fresh. His back catalogue is one of the most dazzling of any director working today, but he rarely watches his own movies. “I have that fear that if I start looking at my films, I’ll be Gloria Swanson in Sunset Boulevard, in her living room, watching her own movies, reminding herself of the glory days, and I don’t want to be there. Ever. So I only look back when my grandkids want to see ET.”

His next film, Ready Player One, dips into the world of virtual reality and promises to be a pop culture sci-fi extravaganza. Another project he hopes to complete is a drama based on the case of Edgardo Mortara, a Jewish boy forcibly raised by Christians in 19th century Italy. Spielberg says he was attracted to the story by the theme of “proselytising” and “how easy it is with young people to bend their minds in a direction that they were never intended to go.”

He has cast Mark Rylance, with whom he worked on Bridge of Spies and The BFG, and Isaac Eshete, but is stuck finding his lead, despite several open casting sessions in London.

“If I can find that amazing miracle child, between the ages of six and eight, but looks more six than eight, I’ll make the movie,” he says. “I have spent a year looking for this kid, because the whole movie rests on this kid’s shoulders. When I work with kids it has to be the right choice.”

Spielberg’s track record of finding child actors for major roles suggests he will eventually succeed. In the meantime, his half century in the business is looming on the horizon. Does he know how he’ll celebrate it yet?

“I will probably go off my diet and have a hot fudge sundae,” he says smiling. “That’s all I need.”

I suppose when your net worth is $3.6 billion (according to Forbes magazine), it’s the simple things in life that matter the most.

The Post opens on January 19