Tobey Maguire did not receive an Oscar nomination, despite being nominated for a Golden Globe, for portraying Captain Sam Cahill.
Maguire plays the "good" brother and Jake Gyllenhaal plays the "bad" brother Tommy. That is technically good casting in that they can be taken as brothers as they both fit a similar type and it is no surprise to learn that Gyllenhaal was considered to replace Maguire as Spider-Man when it was in question whether or not he would reprise the role in Spider-Man two. The comparison between the two is problematic in that I would not hesitate in saying Gyllenhaal is simply the much more talented Maguire as Maguire has a tendency for a blandness in his performances. This blandness is not avoided here either even though this ends up being a very emotionally charged role. The beginning of the film though is just setting up the family dynamic with showing Sam as a devoted soldier but also a man who is very loving toward his children and his wife (Natalie Portman).
Maguire sorta fails in both of the points early on. He just never really seems like a man who was ever a soldier here and he does not do anything to really suggest anything about Sam early on. He's very paint by numbers just kinda going through the motions of any scene. He does not show really a command you might expect from a respected Captain like he probably should. He also does not strike up enough chemistry with either Portman or the two child actors playing his daughter. Also he does not really strike anything notable with Gyllenhaal, in fact it does not seem like he really is trying all that hard in that case whereas Gyllenhaal really is trying to bring something between the brothers. Unfortunately there is not anything that comes from Gyllenhaal's effort because Maguire does not reciprocate, and despite being called Brothers Maguire and Gyllenhaal don't share that many scenes together.
Everything changes though once Sam goes off to Afghanistan and his helicopter is shot down, but he survives although he is captured by enemy forces. They take him and a fellow soldier and proceed to torture them to elicit "confessions". Maguire in these scenes isn't terrible in that I do think there are glimpses of expressing the severity of the situation and the change in Sam's mental state. Maguire never pulls the emotions together enough to make these scenes truly harrowing. There is still an inadequacy at times as he does not bridge the transformation from one to another well enough. There are moments where he seems to have it in portraying Sam's descent, but then others he seems lost in the scene. This is especially true for the pivotal scene where his captors force him to kill subordinate. Maguire does not build the moment instead going to some standard yelling and just undercuts what should be an essential character moment.
Sam comes back and is obviously a bit off as he no longer seems the same man, and is clearly suffering from post traumatic stress syndrome. Maguire actually has one really strong scene where Sam accuses Tommy of sleeping with his wife. Maguire is very effective in showing the unsettled nature of Sam and just the cold nervousness of the man. Maguire in that scene realizes the state of Sam rather powerfully, unfortunately this is not the case for the rest of the scenes. Maguire just seems to go on a blank stare autopilot that fails to bring his conflict to the forefront. He stays this way until an extreme breakdown that occurs. It's a loud scene and really it comes off as much more of him acting loudly rather than bringing the natural breakdown. One reason is he did not build to this well enough, other than that one scene with Gyllenhaal, but as well it is just insufficiently acted in a base sense. Although the ending is particularly weak this isn't a terrible performance as there are moments where Maguire finds the character, but those moments are the exceptions.