When Glenn Greenwald's partner, David Miranda, was detained in Britain last year, Edward Snowden spent the day in contact with the reporter, an "extraordinary, and poignant, role reversal," Ed Pilkington writes about an interview with the journalist a year after Snowden's revelations.

"When he heard David was detained, Snowden was so enraged and concerned, which shocked me," Greenwald says. "His own situation was very uncertain at the time – he's facing 30, 40 years in prison if he ever comes back to the US, and yet he was supportive of what I was going through. That's when I realised we were bonded in an eternal way to the same cause, which most journalists don't like to admit, but I have no trouble admitting – we work to the same ends. We have a bond, a human bond."

Bond or no bond, it strikes me as interesting that in the book, as in conversation, Greenwald only refers to his interlocutor as "Snowden", never by his first name. What's that about?

"It's the weirdest thing. I cannot call him Ed. For the longest time we referred to him as 'the source'. And then when I met him, I never used his name. It just rings false to call him Ed."