Another excellent adventure for Keanu

Bill & Ted star Keanu Reeves goes behind the camera for a new documentary, Side by Side. He tells James Mottram about what's clearly a passion project

Like the subject of his new documentary, Side by Side, Keanu Reeves seems to be at a transitional stage in a 28-year career that has seen him navigate between blockbusters (Speed, The Matrix) and more idiosyncratic projects. He may not admit it, but despite looking youthful for his 47 years, he must secetly know that playing the leading man – as he will do later in the year, in the samurai tale 47 Ronin – has a shelf life.

So it's no surprise he's gone behind the camera for the first time, producing Side by Side, a film he originated with its director Chris Kenneally. It is an insightful snapshot of his industry at a crucial crossroads in its history. Examining the history and process of both digital and photochemical film, Side by Side weighs up the merits of both at a time when digital cameras look set to make obsolete the more traditional methods of shooting on actual film stock.

Reeves presented Side by Side at the international film festival in Berlin, where we meet in the Glashütte Lounge, on the 21st floor of a building in the Potsdamer Platz, offering a stunning panoramic of the city. It's shortly before 1pm, and Reeves is already reaching media saturation point. "OK, one more before lunch," he tells the PR, before she ushers me across to the cordoned-off area where he's sitting on an uncomfortable-looking designer sofa.

My past encounters with Reeves have found him an awkward interviewee, unwilling to articulate anything but the barest of information. But today, he's pumped with enthusiasm for what is clearly a passion project. In a suit, shirt, and walking boots presumably chosen to combat the snow outside, he even raises a smile when I ask if he's found any time to visit Berlin's world-famous clubs. "Clubbing? I'm too old for that."

Side by Side began when Reeves was working on the 2010 low-key thriller Henry's Crime, in which he starred. "This experience was really based upon interest," he explains. "The cinematographer Paul Cameron was showing me these images on this 5D digital camera, and we were looking at the digital image and the photochemical image side by side and I was like, 'Film is going away? Woah! What's happening? Is this the end of film? What is going on here?'"

At that point Reeves partnered up with Kenneally, who was post-production supervisor on Henry's Crime, and decided to document this transitional moment in film. "From the very beginning, Keanu had a strong curiosity to know how everything worked, while we were in post," Kenneally tells me. "How does the lab work? Where does the film go next? And then how does it go to digital? He really dissected it all and wanted to know the process. I've seen him do that with the camera too: he breaks things down. He's just interested in things and how they work."

If the film carefully sidesteps the highly subjective question of which format is better, the roll call of interviewees is illustrious. Directors Martin Scorsese, David Lynch, Steven Soderbergh, Christopher Nolan, Lars von Trier, Robert Rodriguez, David Fincher and James Cameron all appear,

It doesn't stop there, with lesser-known, below-the-line talents given equal billing. "As I started learning more about the subject," notes Reeves, "it would be like, 'Well, why aren't we talking to them?' So we'd go, 'Let's go and see if we can meet them!'" Most are not ex-collaborators, though Reeves does call on the creators of The Matrix – the Wachowski siblings – to discuss what became a groundbreaking moment in film-making.

So does he view The Matrix, with its revolutionary digital effects, with different eyes now? "I think you have to because time has gone by," he replies. He cites the influential "bullet time" sequences, where the camera appears to rotate around a slowed-down image. "It was a digital moment. And that can only be put in the context by the passage of time, generally speaking. John Gaeta, who did the visual effects, knew what was going on, and so did the Wachowskis – but I didn't."

While it's not addressed directly in the film, we move on to the subject of actors being replaced by digital avatars in the future. "I think technologically that would be possible, but I think it would depend on the film-maker, in terms of whether they want reality or not," says Reeves. He admits he's concerned about the possibility of manipulating an actor's performance digitally – "like putting tears in your eyes when you don't want tears".

Reeves, of course, has enjoyed his time with a few great film-makers but did meeting all these other cinematic legends reinvigorate his desire to act?

"I wouldn't say that. I've never had that moment where I felt 'I don't want to do this anymore.'"

'Side by Side' and '47 Ronin' will open later in the year