[REC] and [REC] 2--Sleep Tight is definitely worth a watch, but not a rewatch. It's like a really great piece of gum.
The plot reminds me of something like Roman Polanski's Repulsion, with the psychological aberrations shifted from the woman to the man, from prey to stalker. On the psychological side of things, we the audience aren't dragged down a rabbit-hole of mental breakdown in the usual fashion, but, rather, the psychological aspect remains simple: this guy César is unhappy and wants everyone to be unhappy, especially the vibrant, young, fresh, and (most importantly) eternally optimistic Clara. So, he has resolved to bide his miserable time trying to make Happy Clara crack and become Unhappy Clara.
Again like Polanski (Knife in the Water, Repulsion, Rosemary's Baby), the setting is very limited. Aside from a few scenes in a hospital (which help to give some depth and validity to the character's madness), every frame takes place at the apartment building. While not as restrictive as, say, Buried, which takes place entirely inside a coffin, having so much of the film take place inside the apartment enhances the purpose of the film tremendously for the viewer. You feel the banality and misery that César feels working there, doing the same things day in and day out. You get a sense of suffocation and claustrophobia--that subtle feeling that comes from wanting to take a step outside for fresh air and not being allowed to. And you don't get a lot of extraneous fluff (unnecessary characters and subplots or lengthy sequences for runtime padding)--the camera, the story sticks with the contrast of the two main characters.
The roles are simple and well fulfilled: Luis Tosar plays the perpetually unhappy César, and Marta Etura the carefree, beautiful Clara. The contrast, especially with the use of English-language music, is well depicted. The set design of César's flat versus Clara's flat; the drab gray coat César must wear for his job as a sort of concierge-super versus the fun, artsy attire Clara appears in. The only acting-related irritation I experienced was the interaction between these two when their paths would cross in the lobby of the apartment building in the mornings. The best I can explain my irritation is this: it seemed too obvious that Marta Etura was an actress in a movie and thus aware of the fact that she was supposed to act like she had no idea César was a creep who was "stalking" her. But the good news is that these on-screen interactions didn't take up much of the film.
In the end, Sleep Tight was an above-average, though not extraordinary, thriller. Since we get more focus on these two central characters, with limited exposition, the film gets creative with showing us how, exactly, this guy is acting on his obsession with making Clara unhappy. And, really, they couldn't have picked a better opening scene, because I was really caught off guard and hooked into sticking around to figure out what's going on. What ensues is a film that is well thought out by a writer who takes time thinking creatively about what can be done with very little. This is one of those films where small seeds are planted in the beginning that grow and are intelligently used to bring the film to a close. A great start to the new year!