The producers of the movie took several risky liberties with the story. They keep the premise, that the remains of humanity are living in a walled city under siege by giant beings that eat humans. They jettison much of the rest of the story. Unfortunately, these risks don't pay off.
It's understandable that a long series would have to cut quite a bit of material to fit into a movie, even two movies. Still, it is inexplicable that they would cut the most popular character, Levi, among others. Worse, the missing characters are replaced by new characters we're given no reason to care about. If there was space for a character there, why not use the original?
Of course, the characters that remain are butchered. Their motivations are either missing or replaced by something far weaker. This means that most of the characters make no sense, despite character being a strength of the original. (Jean, I think, suffers the worst inexplicable motivation.) And what is done to Mikasa (Mizuhara Kiko) is a tragedy. In the Attack on Titan manga, she's a force to be reckoned with. She's the best fighter and quite smart. Here, she sits and cries while a titan approaches and makes no effort to run or hide of anything.
|Trailer still of Mikasa-in-name-only|
If you've been curious about giving this story a try, don't start with the live-action movie. This is a sad cash-in on a popular franchise. I'd recommend starting with the anime, then the manga. (Isayama Hajime's art is rough, making it difficult to get into the story at first.) As it turns out, the book is better transcends language and culture.