Daniel Ellsberg, the US whistleblower celebrated in Steven Spielberg’s new film, The Post, was called “the most dangerous man in America” by the Nixon administration in the 70s. More than 40 years later, the man he helped inspire, Edward Snowden, was called “the terrible traitor” by Donald Trump, as he called for Snowden’s execution.

The Guardian has brought the two together – the most famous whistleblower of the 20th century and the most famous of the 21st so far – to discuss leaks, press freedom and other issues raised in Spielberg’s film.

Starring Meryl Streep and Tom Hanks, The Post deals with Ellsberg’s 1971 leak of the Pentagon Papers, which revealed presidents from Truman to Nixon lying about the Vietnam war. It deals, too, with the battle of the US media, primarily the Washington Post and the New York Times, to protect press freedom.

During a two-hour internet linkup between Ellsberg in Berkeley, California, Snowden in Moscow and the Guardian in London, the whistleblowers discussed the ethics, practicalities and agonised internal debate involved in whistleblowing and how The Post has a special resonance today in Trump’s America.

They are worried about Trump’s assault on press freedom and express fear that journalists could be indicted for the first time in US history. And they are alarmed by the prospect of a US nuclear strike against North Korea, urging a new generation of whistleblowers to come forward from the Pentagon or White House to stop it.

Matthew Rhys as Daniel Ellsberg in The Post.

“It is madly reckless for this president to be doing what he is doing. Whether he is, in some clinical sense, crazy or not, what he is doing is crazy,” says Ellsberg. His book based on his experience as a defence analyst and nuclear war planner, The Doomsday Machine, was published in December.

Back when Snowden was debating whether to leak secret NSA documents, showing the scale of government mass surveillance, he found inspiration in a 2009 documentary, The Most Dangerous Man in America: Daniel Ellsberg and the Pentagon Papers. After Snowden handed over material to journalists in 2013, Ellsberg was among the first to express support and the two became friends, with Ellsberg visiting Snowden, who is living in exile in Moscow, in 2015.

They have a shared interest in press freedom. Ellsberg cofounded the US-based, not-for-profit Freedom of the Press Foundation, which helped organise the linkup. Snowden, who also serves on the foundation’s board, devotes much of his time in Moscow to developing tools that help journalists protect their communications and sources.

Ewen MacAskill: How has whistleblowing changed in the 40-plus years between your leaks? One of the striking images from The Post is of leaked documents having to be laboriously photocopied, in contrast with today.

Daniel Ellsberg: Certainly, the ability to copy and release hundreds of thousands of files or documents, as Chelsea Manning did, or millions of pages, as Ed Snowden did, was quite impossible then. I was using the cutting-edge technology of the day, Xerox, to do what I did do, which was to copy 7,000 “top secret” pages. That could not have been done before Xerox.

So, in a sense, it is easier to get the truth out now than it was in my day. It took me months of effort – copying night after night. On the other hand, unless you are an expert like Ed or Chelsea, their ability to trace who has done the leak is probably greater than it used to be. You can’t do it safely. As I understand it from Ed – you tell me, Ed, if I am wrong here – you felt with your counterespionage expertise you probably could have done it anonymously, but you chose not to do so. But others would be more likely to be caught.

Edward Snowden in 2014. Photograph: Alan Rusbridger/The Guardian

Edward Snowden: First of all, a small correction for the record. Dan said I gave millions of documents to journalists. The figure is thousands. The point between the period of Dan’s activities and mine is the expansion of reach of a particular source who witnessed some wrongdoing. In Dan’s case, what he had in his safe was the limitation of his reach. My reach was across a network rather than the confines of a safe … And what this ultimately results in is a dynamic where a particular employee can plausibly – in fact, not just plausibly but demonstrably – have more access at their fingertips than the director of an office or a unit or a group or an agency – or perhaps even the president.

EM: Another difference is Ed was able to operate solo whereas you, Dan, needed a team of volunteers.

DE: There was a kind of pickup crew, largely graduate students at Harvard, who helped find us places to stay and helped transport these papers. They were known as the Lavender Hill Mob, after the British movie in which a random bunch of petty criminals carry off a great heist. When my book, Secrets: A Memoir of Vietnam and the Pentagon Papers, came out in 2003, I wanted to tell their story, but they still did not want their names known because they thought the attorney general, John Ashcroft (who was in George W Bush’s administration), might have imprisoned them. I was signing books and people were giving me little cards with the inscriptions they wanted me to write. A little card appeared: “To the Lavender Hill Mob.” And there was someone I had not seen in 40 years.

EM: How do you feel about your portrayal in The Post?

DE: I am portrayed by a very handsome actor, Matthew Rhys. So my wife and I are quite satisfied with that. The movie is incredibly timely because we are dealing with a president who lies as he breathes, unapologetically. Also, a president who is contemptuous of the press. Nixon called the press the enemy. And Trump’s people say it is the opposition party, which is of course the enemy. When I was watching the film’s premiere, I was thinking: this is a question of freedom of the press.

EM: How about you, Ed, your portrayal by Joseph Gordon-Levitt in Oliver Stone’s 2016 movie? Did it have the impact you hoped for?

ES: I loved Joseph Gordon-Levitt. One of the funny things is they have trapped me in time over the course of my existence as the way I looked when I came forward, always wearing glasses, kind of nerdy. But the funny thing is most of my life, even today, I never wore glasses. I wear glasses in professional settings not because I love the look or whatever. For all the complexities of the film, which was basically slapped together in a hurry because events were developing around the world, they got the core of it, the most important part of it, right, which is what is happening with mass surveillance and why it matters.

Daniel Ellsberg (left) with his co-defendant, Anthony Russo, outside the federal court in 1973. Photograph: AP

When we talk about the impact that it produced in the public, I see responses to this day from people who had seen this but who have not seen Citizenfour [Laura Poitras’s 2014 documentary about Snowden], which is the real documentary. And they just had not understood the issue. News reports had not reached them, but cinema did. They might not be the type to watch documentaries but they are the type to watch a drama. I think that is an incredible thing.

EM: What motivated you to take the final step in becoming a whistleblower?

DE: I would not have thought of doing what I did, which I knew would risk prison for life, without the public example of young Americans going to prison to make a strong statement that the Vietnam war was wrong and they would not participate, even at the cost of their own freedom. Without them, there would have been no Pentagon Papers. Courage is contagious. I have heard you say, Ed, that The Most Dangerous Man in America was a factor in encouraging you to do what you did.

ES: That is absolutely true. While I was weighing up whether to come forward or not – and this was an agonising process because it was certainly life-changing – I watched that documentary. Dan’s example, hearing the arguments from someone who has lived through this, it helps prepare someone to make that jump themselves.

I read, Dan, that you were described, maybe it was by Nixon, as self-righteous. But there is in whistleblowing a kind of righteousness that is required, even self-righteousness. Everything in your head, in society and everything we have been indoctrinated into believing is screaming: “Don’t do this!” And yet there is some voice that builds over time that has to persuade a person that they do not just have the right to do this but a responsibility to do so; to make the move that will certainly burn their life to the ground. But, theoretically, the wellspring of hope that is the motivational force behind this is that it will redress some wrongdoing.

Joseph Gordon-Levitt in Snowden. Photograph: Allstar

EM: Is the threat posed by Trump greater than that posed by Nixon?

DE: I believe this president will indict journalists, which has not happened yet in our country. We fought a revolution to avoid that. And we have not yet broken that first amendment, which protects press freedom, in our constitution. But this president is likely to do so. The climate has changed. And that was true under Obama, who prosecuted three times as many people for leaking as all previous presidents put together – he prosecuted nine. I think Trump will build on that precedent. He will go further and do what Obama did not do and directly indict journalists.

EM: Is the WikiLeaks founder, Julian Assange, holed up in the Ecuadorian embassy in London and fearful of extradition to the US, one of those at risk?

ES: Julian’s best defence, perhaps his only enduring defence, is that he is a publisher and has never even tried, as far as we are aware, to publish something untruthful. There are lots of criticisms, many of which are legitimate, to be said about his political views or his personal expressions or the way he put things or his agenda. But ultimately the truth speaks for itself.

DE: Assange is in danger. There are those who say that Julian does not have to fear extradition if he came out of the embassy and served a brief sentence, if anything at all, for violating the rules. I think that is absurd. I think Britain would ship him over here [to the US] in a minute and we would never see or hear from him again … under Trump, he may well be the first journalist in this country to be indicted.

EM: What about whistleblowing to prevent a US attack on North Korea?

DE: I am sure there are thousands of people in the Pentagon and the White House who know an attack on North Korea would be disastrous because they have estimates and studies that show the outcome of a supposedly limited attack would be catastrophic in terms of hundreds of thousands of lives, millions of lives and what comes after.

ES: What would you say, Dan, to the next whistleblower, who is sitting in the Pentagon? They have seen the attack on North Korea planned, they have seen the consequences and it can be stopped.

DE: They have, of course, something I did not have then, which is they can go directly to the internet. And that is not something I would advise them to do. I think that, let’s see, in your case you went to the Guardian, you did not put the stuff on the net directly as you could have done. I think you did the right thing … If the New York Times does not do it, if the Guardian does not do it, you have the internet to go to.

EM: Was whistleblowing worth it?

DE: I once read a statement by Ed Snowden that there are things worth dying for. And I read the same thing by Manning, who said she was ready to go to prison or even face a death sentence for what she was doing. And I read those comments and I thought: that is what I felt. That is right. It is worth it. Is it worth someone’s freedom or life to avert a war with North Korea? I would say unhesitatingly: “Yes, of course.” Was it worth Ed Snowden spending his life in exile to do what he did? Was it worth it for Manning, spending seven and a half years in prison? Yes, I think so. And I think they think so. And I think they are right.