IT'S early afternoon in London's Soho. Ensconced in a five-star hotel, Daniel Radcliffe is surrounded by empty coffee cups, a tobacco pouch and a scented candle. Dressed down in jeans and sweatshirt, he's full of beans, even if the proverbial fly on the wall to our conversation might get the shock of its little life. Farts, hydraulic penises, Edward Snowden and white supremacists ... these are not the topics you expect to be discussing with the former Harry Potter star.

Where to start? Radcliffe is starring in two radically different indie movies, playing an FBI agent infiltrating a far-right gang in Imperium and – there's no other way to put this – a farting corpse in Swiss Army Man. Let's start with the latter, if only because you probably just had to re-read that last sentence. Paul Dano plays Hank, a shipwrecked loner on the verge of killing himself when Radcliffe's lifeless body washes ashore. Soon enough, this cadaver – later named Manny – starts breaking wind; gradually, it begins to speak, twitch and even endure involuntary erections.

Radcliffe, 27, calls these bodily functions "stand-ins" for the things we're all taught to be ashamed of. "And that's the message of the film – shame keeps us from love," he says. But even he blushes when he starts to discuss the penis sequences. Modelled (apparently) on the prehensile appendages sported by the Tapir, the South American warthog, Manny's member was manoeuvred by hydraulic levers. "It was this huge rig that this guy was moving around." Radcliffe says. "It was pretty funny."

It’s no surprise that when it premiered at the Sundance Film Festival earlier this year, Swiss Army Man became known as the "farting boner corpse" movie. Radcliffe proclaims it rises far above these rather puerile beginnings. “It's about what it means to be human," he says. "My character is getting a super-accelerated version of the human experience in an hour." As Hank finds the will to live – think of Tom Hanks's movie Cast Away with Radcliffe as the volleyball – what emerges is the sweetest, strangest bromance you'll ever see on screen.

Admittedly, Radcliffe's turn in Swiss Army Man is not entirely out of character. Since bidding adieu to J.K. Rowling’s boy wizard Harry in 2011, this West London-born son of two agents has played Beat poet Allen Ginsberg in Kill Your Darlings, a boy with protrusions sprouting from his head in Horns and the hunchbacked Igor in Victor Frankenstein.

"I've always felt I've been really lucky to work on the films I've got to work on since Potter," he says. "When you start doing roles that are a little weirder, like Horns and Swiss Army Man, then weird begets weird. You get a reputation."

More straightforward (albeit terrifying) is Imperium, in which Radcliffe plays Nate Foster, an idealist who goes undercover in a white supremacist gang to seek out information on a possible terror attack. With Radcliffe even shaving his head on screen, it meant an intense period of research, anonymously trawling internet sites belonging to extreme far right groups. "The message boards were the bleakest bit of the whole thing,” he nods. “That's where you'd go to get the raw, uncut diatribe.”

With the script based on former FBI agent Michael German's book Thinking Like A Terrorist, Radcliffe spent weeks devouring it, alongside white supremacist literature. "I thought, 'I can't keep carrying these around with me everywhere, in case I go to an airport!'" Couple that with his internet search history, as part of his research, and he's arguably being watched for all the wrong reasons. "Between that and the fact I did a play with Edward Snowden this summer, I'm pretty sure I'm on all the lists!"

Back in July, Radcliffe headlined a Donmar Warehouse production of James Graham's play Privacy, featuring alongside NSA whistleblower Snowden (albeit on screen) reciting lines from The Tempest. They met via a "heavily encrypted" web-chat. "As I suspected, I really liked him. It's very funny...he is a historically incredibly significant person already. It's going to be a huge moment when people look back on this time and what he did is going to be viewed as a very big deal, and he's just a normal person...who did a very brave thing, and is now suffering for it."

Having also ventured to Colombia and Australia to shoot Jungle – the story of a Bolivian trek that goes awry – Radcliffe has worked wonders to develop a post-Potter career. Understandably, he won’t be going to see Harry Potter and the Cursed Child, the two-part stage show currently in London's West End that revives Harry's adventures as an adult. "I have this fearful image of me watching the play and people in the audience watching me to see what I'm thinking about the play," he says. "And I wouldn't ever want to be a distraction to what was happening on stage."

Still, it’s a statement that rather sums up the difficulties of playing such an iconic character. No wonder he can't say whether he'd ever return to the role in his later years. "I feel like everyone is asking about it, but I don't think it's a real thing," he says, claiming (possibly naïvely) that he doesn't believe studio Warner Brothers are plotting a Cursed Child film adaptation. He will, though, go and see Fantastic Beasts and Where To Find Them, the forthcoming Potter spin-off/prequel movie. "That's something I can do very easily without people noticing." Well, almost.

Imperium opens tomorrow. Swiss Army Man is released on September 30.

JAMES MOTTRAM