Richard J Evans was regius professor of history at Cambridge University from 2008 to retirement in 2014. In the millennium year, he served as the expert witness for Penguin Books in the libel case brought unsuccessfully by Holocaust-denier David Irving. Evans’s three-volume history of the Third Reich, completed in 2008, is the definitive account. A new book, Hitler’s People, re-examines that history through the life stories of prominent Nazis and supporters of the regime.

After completing your trilogy on the Third Reich, why did you want to return to this material now?
There were several reasons. One was that the biographical approach to German history became very unfashionable – because historians were wary of the Nazi cult of personality. But I think around about the turn of the century, the biographical approach came back; the big pioneer there was Ian Kershaw’s wonderful biography of Hitler. And also that approach coincided with the rise of strongmen and would-be dictators in Europe, and also, of course, more recently in the States. So it has become urgently important to study why people, again, at every level, have started to depart from democratic norms.

You’re so steeped in this history, obviously, but this was also a voyage of discovery for you…
When I was involved in the Irving libel case, as an expert witness, the lawyers asked me for a good, detailed, up-to-date history of nazism. And I actually realised the really good ones were out of date. So I decided to do one myself. The last of that trilogy came out in 2008. Going back to it 15 years later gave me a chance to use this new perspective, but also take into account all the masses of new source material that have come out since the turn of the century – for example, the 32 volumes of Goebbels diaries.

In the intervening years, you’ve done quite a lot of work on conspiracy theories. Why is that?
Some historians have spent an entire lifetime working on nazism. But I think you have to find a way to ward off the depression that inevitably affects you when you study humans behaving in the worst possible way. I wrote a couple of other books, one of which was on counterfactuals.

Did you believe that the Irving trial might put an end to Holocaust denial?
I thought that we had discredited Irving and we had established the factuality of the Holocaust beyond any reasonable doubt. But then within a short time, along came the internet, along came Holocaust denial websites and the removal of the gatekeepers such as news editors. Denial has spread as a result.

We used to think of conspiracy theories as being eccentric or benign – less so now…
Yes. Or as Donald Trump’s spokesperson described provably false information, “alternative facts”. For journalists and historians and scientists, that’s not acceptable. We are engaged in a search for the truth. We can’t compromise on that.

A simplistic question, but do you feel that you know Hitler and his circle?
Well, the reason I wrote the book is that I felt, despite all that work, I didn’t really know these people very well. I found out things about them that hadn’t been given that much prominence. For example, there are several examples of prominent Nazis who, as children or adolescents, watched their father lose his position in society, or leave Germany in disgrace. I found that very striking.

Did you watch Jonathan Glazer’s film The Zone of Interest?
I saw the film and I actually helped Martin Amis with the original book on which it was based. He asked me to go through it and I found 50-odd mistakes. He was a wonderful writer, but I think the whole thing is slightly morally questionable, really.

In the sense that it departs from historical fact?
Yes, and the film actually departs from the book in a number of interesting, significant ways. I think there’s a problem in that the film portrays the central commandant figure as a person who has this sort of happy bourgeois home life. The autobiography of Rudolf Höss, the commandant of Auschwitz, suggests that was far from the case. Even so, it’s a very powerful, thought-provoking film.

There were lots of parallels being drawn during the political upheavals of 2016 with the 1930s. It seems, knowing what we know about Trump, those parallels are more pertinent this time around…
Yes, I’m very, very concerned. Mark Twain said that history doesn’t repeat itself, but sometimes it rhymes. Trump is not Hitler. Nazism was militaristic, hellbent on conquering other countries. Trump is an isolationist. But still I think he and his supporters are a real threat to democracy, in their fanaticism and denial of truth.

What are you doing next?
Well, I want to try a more cheerful subject: I’m planning to write a book about pandemics. I’ve had enough of Nazis.

Is this your final word on them?
I hope so. I grew up in east London. I remember going into parts of Hackney in the early 50s and seeing huge bomb damage. My interest in history was awakened by the question: who on earth has done this? And here I am, 70 years later, still trying to answer that.

I imagine that your biography of Eric Hobsbawm (published in 2019) not only provided a model for that kind of longevity, but also some inspiration to never stop.It’s funny, I was elected a fellow of the British Academy and the first thing they do is ask you for material for your obituary. They also asked me to write something about Eric. I went to his house and I found an absolute treasure. He never threw anything away and he lived to be 95: letters, diaries, correspondence. He was not only a great historian, he also took part in and observed the major events of the 20th century. So that was certainly a distraction!

You mentioned the danger of getting depressed when immersed in these horrors – how do you avoid that?
First, I have a good natural constitution, I don’t easily get depressed. And then, of course, I do things I love. I play music, go for walks with the dog – and to concerts and plays. In terms of books, I read Dickens, and, of course, I always find PG Wodehouse a reliable light relief.

• Hitler’s People: The Faces of the Third Reich by Richard J Evans is published by Allen Lane (£35). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply

• This article was amended on 4 August 2024. The names of the historian Eric Hobsbawm and the writer PG Wodehouse have been corrected, and an earlier version referred to the diaries of Rudolf Höss, the commandant of Auschwitz, when it meant to refer to his autobiography. In addition, Donald Trump’s spokesperson used the term “alternative facts” to describe provably false information, rather than conspiracy theories as an earlier version suggested.