When Billy Bob Thornton sits up against the tree and nestles his rifle against his chest to get a good night's sleep, it's an homage to Grizzly (1976) when Richard Jaeckal's character did the same thing.
Adrien Brody was heavily involved with this early on, wanting to star and produce. He met with Gerard Butler and discussed with him the idea of them playing brothers in the film, but he eventually passed.
The grizzly bear is Bart the Bear II. He is an interior Alaskan grizzly bear.
The cast went back to Vancouver in October for additional filming. Rumors of going back to the original ending from the original screenplay which had Adrien Brody attached to it is what they used.
Missy Peregrym auditioned for a role, but the producers went in a different direction. Coincidently, or not... she went and made a similar themed bear film with her television show friends called Backcountry (2014), that has many of the same scenes from the original screenplay "Red Machine".