Best word to describe this movie is "thin." The trailer gives the setup, but there's no follow through. Nothing is particularly believable or smart and there's little plot, dialog, or character interaction. Questions that should be asked aren't. Questions that no one really cares about are lingered on too long. The main character dominates the screen time and spends most of the movie alone and silent, so that conveniently leaves out the need to flesh out his theories and what he's thinking and we're just left to guess--no doubt the director thinks this is a clever style because it puts us in the same mindset as the main character. Actually, it just put me in the mindset of wanting to go to sleep.
The main character is a stony hired killer, so there's no one to root for, and it doesn't take too many lingering shots of him furrowing his brow to convey the wheels of his genius brain are turning while conducting surveillance of mundane events until you stop caring. Slogging through to the ending adds little, so you might as well just move on when the boredom gets intense. There's really not any "twist" at the end, as some say; what you expect to happen pretty much does.
Ironically, one of the main messages of Accident seems be that one should be wary of the human tendency to read too much into things, yet that's just what the reviewers who trumpet this movie as one of the best of the year have done, when this actually yet another triumph of atmosphere over substance that bypasses viewers' ability to think critically by implying things that never materialize and various other manipulations that leave you feeling used at the end. Overheard (2009), also with Koo, springs to mind as an example of a better movie of this stripe.