The character development slows the movie down, but is wholly necessary to build the story. Jeremy Renner (Arrival, The Avengers, The Bourne Legacy) and Elizabeth Olsen (Avengers: Age of Ultron, Godzilla) are at the forefront of the mystery, but other actors contribute great performances as well. Graham Greene (The Green Mile, Die Hard with a Vengeance), Gil Birmingham (Hell or High Water, Rango), and Kelsey Asbille (The Amazing Spider-Man) bring important secondary characters to life, helping to create a diverse screen for Sheridan. I really enjoyed Graham Greene as the police chief on the reservation. Without his character, many viewers would struggle to understand why the mystery is hard to solve.
While the main action does not come until the second half of the film, the wait is totally worth it. An explosive final act gives the audience what they waited for. There is one fantastic transition towards the end of the film that gives the audience a complete explanation for the mystery. Rather than having a smartass character explain the mystery, Sheridan effectively shows the discovery. Just as the characters figure out what happened, the audience has the realization. And of course, as any intriguing mystery resolves, there is a shoot-out to give audience members the action sequences they paid for.
The Bottom Line: Wind River is a uniquely diverse mystery with a slow start and explosive finish.