Continuity mistake: In one of the scenes Hopkins is trying to make what I think is a fish hook or something. He's obviously been in the wilderness for a while so he's not clean shaven but he isn't exactly beardy either. However, during the scene with this fish hook his beard changes from moderate to bushy to moderate again within seconds.
Continuity mistake: When Charles is showing Steve the constellations, the sky is pitch black. Yet in the next scene when Charles goes down to the shore to Bob, the sky has brightened considerably. No indication is given that a good deal of time has passed between these two scenes.
Continuity mistake: When Charles is performing CPR on a man who drowned when the plane went down, in one angle he's pinching the nose closed, as he should, but from a different angle he's not.
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 ★