More than anything else, I Am Legend is a testament to Will Smith's considerable star power: for much of the film, he's the only identifiable character, as in Tom Hanks's similar solo turn in Cast Away (Smith at least has the benefit of an eerily de-populated New York and an expressive dog as support, where Hanks had to rely on a ball as straight man). Smith, a tremendously appealing performer, is also a solid actor, and he's convincing as Robert Neville, New York's last remaining resident, a resourceful man of action. Despite a carefully honed routine, he's prey to his own mind - driven half-mad, on his bad days, by the necessity of sustained paranoia and the lack of human company.
The film is on surest ground while Smith is alone - his daily explorations of the city are suspenseful and strange, whether driving the wrong way up weed-infested Manhattan streets or exploring the homes of those killed by the devastating virus that has swept all before it, save Neville, who is trying to understand his own immunity (Emma Thompson makes a brief, uncredited appearance as Dr. Krippin, the virologist who's to blame; her name is reminiscent of another famous killer who found himself on the wrong end of an earlier technology of global reach). Halfway through, the film switches gears, abandoning much of its subtlety in favour of more mundane (and too obviously) computer-generated thrills; the film's budget doesn't stretch to the kind of authentically thrilling denouement of 28 Weeks Later, with which it shares some thematic similarities, though less filmmaking chutzpah.