The Machinist is Trevor Reznik who works in a heavy industry. For certain reasons, he has been suffering from insomnia for over a year. This has caused him to become inhumanly thin, and also obscured his perception of reality.
I can’t elaborate more in the plot or it will spoil the fun of watching it.
The cinematography is brilliant, similar to the Japanese mystery/horror movies like Dark Water/Ring which build up the element of mystery slowly with the use of environment and darkness in general, unlike the typical Hollywood devices of gore, sudden noises/camera movements etc.
Christian Bale playing the protagonist Trevor Reznik, looks like he just got out of Auschwitz. Surviving on a diet of 275 calories a day over several months, he reduced his weight to an incredible 54 kg, just to look the part! His appearance is totally different, and someone who has seen him in any of his other movies will have trouble recognizing him, just like Edward Norton changed himself for American History X