Benedict Cumberbatch has revealed details of his email exchanges with Julian Assange about the new WikiLeaks film, including Assange's plea for him not to take part in the movie, sent the day before filming began.

Cumberbatch, who plays Assange in The Fifth Estate, told the Guardian he had received the 10-page email from the WikiLeaks founder as he made final preparations to begin filming in January.

"It was a very considered, thorough, charming and intelligent account of why he thought this was morally wrong for me to be part of something he thought was going to be damaging in real terms – not just to perceptions but to the reality of the outcome for himself," the actor said.

"He characterised himself as a political refugee, and with [Chelsea, formerly Bradley] Manning awaiting trial, and other supporters of WikiLeaks who have been detained or might be awaiting detention, and the organisation itself – all of that being under threat if I took part in this film."

Assange has been outspoken in his opposition to the film, which he has described as a "massive propaganda attack" on him and his organisation. The movie is based on two books, by the German technology activist Daniel Domscheit-Berg and by Luke Harding and David Leigh of the Guardian, former collaborators with whom the Australian has now fallen out.

But Cumberbatch, who said he made repeated unsuccessful requests to meet Assange before beginning filming, has defended the project, saying the film "shows his ideas and integrity and self-sacrifice".

The actor, best known for Sherlock, did think twice, he says, when he received the email. "Of course [I wobbled]. The fact that it was coming from the man himself, the day before we started filming? Of course I would hear and feel the protests of the man I was about to pretend to be. I'm a human being."

He wrestled with his reply for four hours, he said. "I said listen, this film is going to explore what you achieved, what brought you to the world's attention, in a way that I think is nothing but positive. I admit to doing work because I'm a vain actor … yet I'm not acting in a moral vacuum. I have considered this, and whatever happens I want to give as much complexity and understanding of you as I can."

Asked about Manning – who was sentenced last month to 35 years in prison for leaking documents to WikiLeaks, after which she announced she wished to live as a woman – Cumberbatch said he was not convinced that the army private should be granted the presidential pardon for which she has now appealed.

"He [sic] did what he did out of a conviction that an alarm bell needed to be sounded. But his superiors might have been right to say to him, it's not your position to be worried about it within the hierarchy of the military organisation, which is why he had to be sentenced. He took an oath, and he broke that oath."

While alarmed by the revelations of state surveillance disclosed by the whistleblower Edward Snowden, the actor admitted he was also ambivalent about disclosing secrets. "If they are saving lives, how can we say that's less important than civil liberties? You don't have any civil liberties if you're dead."