Revealing mistake: When the needle is set in the pool of water in a stump to act as a compass the pool has been carved out with a chain saw very recently and clumps of the wrong moss set around the rim like it belonged there.
Revealing mistake: In the last scene, when Anthony Hopkins and Alec Baldwin are trying to kill the bear, they appear to have sharp wooden spears. As the camera angle changes you can notice that Hopkins is holding a green/brown training pole instead of a natural wood spear.
Chosen answer: Although Bob intended to kill Charles (to obtain his wife and wealth), they unexpectedly were struggling for survival in the wilderness. Working together increases their chances of rescue, and Bob cannot survive without Charles' help. He therefore saves him from the rapids. Plot-wise, this makes Charles think that his suspicions about Bob may be wrong. Also, Bob is not a born killer, and may be hesitant about actually committing murder, particularly someone he had admired and respected. Bob likely planned to kill Charles when rescue was imminent and then claim Charles died accidentally. Regarding the title, because the movie is not based on a book of that name, the filmmakers probably chose it for its marketing value because it sounded dramatic. This is a suspense/adventure movie, and "edge" can have multiple meanings, such as being "on the edge," as to whether or not Bob will carry out his plan. It can also refer to the razor-sharp edge of a weapon, an "edgy" emotional situation, and so on.
raywest ★