James Cameron has been planning his Alita: Battle Angel film for two solid decades, having bought the film rights to the manga in a distant 1999.

But various projects (like a little film called Avatar) got in the way, so eventually he tapped Sin City's Robert Rodriguez to direct the film instead of taking it on himself. That can't have been an easy decision, but Cameron told Digital Spy why he knew Rodriguez was the man for the job – his ability to cleverly cut a third of the three-hour-long script.

"You know, you get a little twinge," he said. "Robert had to jump through some hoops for me, or at least clear a hurdle or two, around sensibility. I thought the easiest way to suss that out was to hand him my 180-page, gigantic, sprawling script, and see how he cuts it down.

20th Century Fox

"The way in which he cut it down was quite elegant. He kept everything I liked, and he knitted things together in a more efficient, more direct pathway. Which is why the storytelling is so clear. When you watch the film, it's crystal clear. There's no murkiness to it.

"It sets up the world clearly. It sets up the characters clearly. Except where we're trying to be enigmatic on purpose, which is in respect to some of the obviously sequel set-up kind of ideas."

A page of script roughly equates to a minute of screen time, making the original script a very Cameron-esque three hours long. The completed film runs at just over two hours.

20th Century Fox

Rodriguez covered his living room with 171 cards representing each scene in the script (with a corner presumably set aside for the 600 pages of notes that came with the screenplay), and had to decide the best way of cutting things down to impress Cameron.

"I thought, you know, people think of Jim as spectacle," Rodriguez told us. "They probably think, 'Keep the spectacle. There's a father/daughter love story. There's a boy/girl love story. One of those has got to go.' They would probably take that out. I didn't. I knew that that was really what he was going to miss. He wasn't going to miss me combining some characters or cutting some action.

"And sure enough, he read it, and he said, 'I played a little game with myself, saying, I bet he's going to cut my favourite scene. There's no way he'd cut out 60 pages. This is going to suck.' And he's like, 'Oh, my favourite scene is here. I bet he's going to cut my next favourite scene. No, that one's still here. So I managed to get all the way to the end, and you didn't cut out any of my favourite scenes. How did you do that? Let's make it.'"

Alita: Battle Angel will be released on February 6.

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