Danny Boyle believes '127 Hours' will be a "challenge" to watch.

The director admits the film - which tells the real life story of climber Aron Ralston, who was forced to cut off his arm when it became trapped under a boulder - may be difficult for some viewers because of the explicit scenes.

He said: "We want it to be a challenge to you guys, to see whether you can sit and watch it. Because there is something that happens in it that is quite tough to watch."

Speaking at Movie-Con, Danny also admits the film - which stars James Franco and will open this year's London Film Festival - was difficult to shoot because they didn't want to use any graphics and keep true to the story.

He said: "Weirdly enough, this guy took a camera with him. He had a video camera. And he left a series of video messages every day. He was trapped there for six days.

"He's never shown them to anyone - well, his mum's seen them and some friends. They're in a bank vault. He did show them to us. It's weird watching them because he records them every day at the same time.

"And he thinks he's going to die, you can see. We didn't do this because we didn't want to do CG. When he runs out of water, which he does after about three days, there's a gap then to him leading the next messages.

"But there's no gap in the tape, it just jumps, because he turns it on and off. But the difference in him is...wow. It's not like weight loss, taking out the calories or carbohydrates. It's actually a weird shrinking of the human spirit, he just vanishes in a way. So we were able to employ that. "