You might be familiar with the concept of the uncanny valley, but a brief refresher: The more realistic a human simulation like an android or CG character becomes, the more we notice the small differences. Eventually we become repulsed. Sometimes we can't even articulate what's bothering us; we just know something feels off, and it makes us uneasy. An often-cited example is the dead eyes of Tom Hanks' various characters in Polar Express.
For the most part, filmmakers are aware of this problem and try to avoid it, making their characters more cartoony or not human at all: Think Caesar from the most recent Planet of the Apes films or the cast of pretty much any Pixar movie. And Alita might initially seem to be another nonhuman character, given that she's a cyborg with a completely machine body: The only part of her that's fully biological, from the start of the film, is her brain. But the movie reminds us constantly that underneath those mechanical parts, she's still an emotionally fragile human girl.
Her cyborg nature is very much central to the plot. Alita, broken and amnesiac, is found abandoned in a scrapyard by a kind cybernetics doctor who replaces her lost mechanical body and adopts her as a daughter figure. She's a stand-in for the child he lost years before, so he's naturally inclined to protect her. But it's tough in a post-apocalyptic city where citizens are often forced to replace their biology with mechanical bits just to survive. And even then, they risk having those ripped away by bandits. Alita is instinctively drawn to the violence of this world, for reasons she doesn't consciously understand at first. It's a story we've definitely seen before, but the fun of watching it this time around is the sheer complexity of this lived-in world. (And, of course, the fights are pretty cool.)
The original Battle Angel Alita comic, created by Yukito Kishiro in the early '90s, was never going to translate well to live action. It's set in a rundown slum called Iron City, populated with cyborgs who aren't always recognizably human. Some are simply replacing lost limbs, others may graft on parts for manual labor and a few may even transform themselves into rolling murder tanks in the hopes of winning the city's favorite deadly sport, Motorball. There's a giant city looming overhead called Zalem, held aloft by a space elevator. It's all visually fantastic and previously could only be depicted with any accuracy through anime: An original video animation was released in 1993 that adapts the first two volumes of the series, but it was never continued any further.
The live-action version has been in the works for nearly two decades -- with original director James Cameron continuously putting it off as he was busy with other projects. Eventually it was passed off to Robert Rodriguez, whose distinct style matches the hyperviolent scenes of the original story. The long delay in making the film helped a lot: The team at Weta applied everything it learned from films like the Lord of the Rings and Hobbit trilogies, Planet of the Apes and, of course, Avatar. However, after Cameron's blockbuster made a huge splash, effects seemed to stagnate: Studios cut costs and underinvested in VFX houses, leading to substandard work. But you won't find any shoddy work here. In fact, there are advances both big and small.
The film gets you to invest in Alita as a real, living human being by imbuing her with a level of detail never before seen in a CG character. Her eyes may be enlarged and her mouth reduced in order to match the iconic look of the manga, but it's something you quickly forget after the first minute. The whole thing was motion captured from Rose Salazar's performance, where Weta was able to pick up the smallest facial expressions: Salazar has a distinct little nose twitch that her computerized doppelgänger also emulates. That's partly how this film manages to avoid the uncanny valley: It has the little quirks we expect to see when interacting with another person. We want to see the little things that make someone different from everyone else.