'Alita' director Robert Rodriguez and stars Rosa Salazar and Christoph Waltz on the cutting-edge film

'It was an ­exercise in learning how to be as bold a filmmaker, as Jim,' says Rodriguez

Christoph Waltz, left, plays Dr Dyson Ido in the film directed by Robert Rodriguez, right. Waltz’s character rescues and reboots the cyborg Alita, played by Rosa Salazar, centre. AP

As a self-confessed fan of the Gunnm comics, Robert ­Rodriguez leapt at the chance to take on James Cameron's stalled Alita project while his friend and fellow director worked on the sequels to the record-breaking Avatar.

The Machete director admits, however, that before he could begin shooting, he spent several long nights with a red pen cutting down ­Cameron's sprawling script.

"When you read a script by Jim, it's not like any other script you've ever read," ­Rodriguez says. "It feels more like a description of everything that he's seen on the screen. Even though he writes it, you see the movie in your head as you read it. I said, 'Wow, I really saw the film you're trying to make; I'm going to make that film. I just need to edit. I'm going to pretend I'm more like an editor than a writer and edit it down to the right length so we can get it produced.'"

The result is a vast sci-fi epic in which the poor now live in Earth's grimy Iron City. But the story is not all dystopian doom and gloom. "The film is about love," says Rosa Salazar, 33, who plays teenage cyborg Alita, discovered on a scrapheap by Christoph Waltz's kindly Dr Dyson Ido, who becomes her surrogate parent. "To be able to have a father is something completely new to her. Watching the two of them experience that is really inspiring, it's really beautiful."

The role sent Salazar on a long journey. She undertook months of physical training as she learnt kick boxing, Muay Thai, Eagle Claw and staff fighting with Keith ­Hirabayashi, who instructed Zoe Saldana on Avatar.

“Basically, it’s a mind-body-and-soul experience and that’s what I didn’t expect when I went in,” she says. “On the set, I was able to do things I never imagined I’d be ­capable of doing because of that training.”

With the wide-eyed Alita an entirely digitally rendered character, there was more for Salazar to endure. Her entire performance was motion-­captured, meaning she had to wear a suit covered in sensors to record her ­movements, with the visual effects team then bringing Alita to life. She even had to ­contend with wearing a helmet ­outfitted with two lightweight HD cameras, designed to capture every facial expression. "That was the main thing to get used to, how to perform and make it all melt away,"
she says.

Yet all of this cutting-edge filmmaking did not get in the way of the cast's ­performances. "Not once was I asked to do anything just for the tech­nology," Waltz says. "It was like intimate ­character-driven drama, where you work on the scenes, on the characters, on making sense of the story."

Rodriguez surrounded ­Salazar and Waltz with a fine cast, including Jennifer Connelly, as Ido's scheming ex-wife Chiren, and Moonlight Oscar-winner Mahershala Ali, who plays Vector, the leader of Iron City.

For Rodriguez, who largely shot on a 90,000-foot set built at his Troublemaker Studios in Austin, he broke new ground with Alita.

"I wouldn't be able to get this movie made without [Cameron]," he says, ­acknowledging the ­support he received from ­the producer. "It was an ­exercise in learning how to be as bold a filmmaker, as Jim."

While some early reviews have been critical – The Guardian described the film as "an air of teen innocence that surrounds an essentially conservative film" – they are rather missing the point. Rodriguez wanted to make a coming-of-age story for younger viewers. "I thought, 'I would love to do a PG-13 type movie with a teenage girl, something my daughter can watch and aspire to,'" he says.

Moreover, it is a tale of what can be achieved when two Hollywood heavyweights collaborate, while checking their egos at the door. "Jim didn't interfere – he made that decision to let Robert make the movie," says Waltz. "This is how it should be."

If only all blockbusters were this harmonious.