Mortensen doesn't just go for one tone as Frank, there is a furry of different elements to this man and he is amazing the way he is able to reflect them. In his first scene he seems to be a reasonable sort having just gotten out of the army, and at least for the moment seems to be able to get along just fine as man. Mortensen is very effective though because he suggests what makes him a man who never really can be his brother in this early scene. There is a spontaneity in his performance that is perfect in showing that Frank can never be like his more reserved brother Joe (David Morse). In this early scene Mortensen portrays it something harmless enough, but has still has it something so instinctual that he can't help but do it.
We Frank later on now looking far more the rebel than he did before in his army uniform having no qualms about his behavior. Mortensen is excellent in conveying basically Frank in his natural state which is rather odd, but entirely fitting. Mortensen makes Frank seem to be a series of contradictions, he is charming to a certain degree, he seems to be enjoying himself as well, but at the same time he seems to be hating himself in just the same way. When his brother informs him of their mother's death Mortensen's reaction is spot in the way he shows that Frank is in some way hurt by this revelation in an understated sadness portrayed by Mortensen, but still he never loses the rebellion in him that forces him to almost shrug it off as nothing.
As the film progresses Frank goes in and out of seeming like he could get over his steak as a troublesome sometimes quite violent streak with his behavior that he moves from seeming reasonable enough but it never seems to stay that way. Even when he seems to be going about a normal life there are moments where he becomes truly reprehensible and irresponsible. Mortensen gets this dynamic across so well by making it always the same man. It is something almost primal in a way as Mortensen never shows Frank as fully one way or the other way he is able to get across the wildness of this personality. Mortensen has Frank fly in all directions at once it would be wrong for Frank to always be wild or to not be at all when trying to be controlled he can never be just one way.
Viggo Mortensen very importantly has terrific chemistry with David Morse through just how understated they are together. The two mostly suggest their relationship through glances but the two get across both the disconnection between the two thanks to their natures but as well their connection due to their history as children together. It is a testament to the strength of their performances that they are able to make the relationship of the brothers the central theme of the film despite the fact that the two actually have a very little dialogue between each other, and most of it is fairly simple except for the point where Joe finally confronts Frank. Due to their underplaying beforehand when the conflict between the two finally comes out it is particularly powerful by the way they so effectively established their relationship beforehand.
This is a strong performance by Mortensen that succeeds in creating this distinct type of personality. Mortensen is very good because he does not go about an easy route for Frank which would be just being a rebel all the time or being black and white in the way Frank can change so easily. Mortensen gets across the instability and the impossibility of such a man to just rest for a moment though by instead being as unpredictable as Frank should be as character. The final result that comes from Frank is made by Mortensen an eventuality even though it seems so sudden sense builds the nature of Frank so well throughout his story. This is not Viggo Mortensen's greatest performance, but it is a great early indication of his considerable talent as an actor.