Keanu Reeves has admitted that his new documentary Side by Side was incredibly difficult to produce.
Side by Side examines digital technology's effect on the film industry over the last few decades.
Reeves told Reuters: "It wasn't easy and it took almost a year to film everyone. We began at the 2010 Camera Image festival in Poland and got a bunch of [cinematographers] there, including all these greats I'd worked with, such as Vittorio Storaro, Michael Chapman and Michael Ballhaus.
"That was our start, and then word-of-mouth spread, and I began contacting some of the directors I'd worked with over the past 25 years.
"So that history together obviously helped get some of the big names on board and we just started building momentum. We ended up getting nearly 150 people and then we had to cut it down for the final movie."
Reeves said that The Dark Knight Rises director Christopher Nolan was the hardest interviewee to track down.
"I actually wrote to him on an old-fashioned typewriter. I think he got a kick out of that and we finally shot him in his trailer on the Batman set in LA," he said.
Side by Side, directed by Christopher Kenneally, opens across the US in the coming weeks.