Ryan Reynolds is the only person we see in the flesh. All of the other performances are either voiceovers or recorded on his cell phone. The whole film is shot from the interior of the coffin. We never see the outside world. The film never repeats a single shot. These all make Buried (2010) one of the most minimalist films ever made.
Ryan Reynolds states that he suffered from claustrophobia towards the end of filming (much like the character he is playing). This was mainly due to the fact the coffin he was in was gradually filled with more and more sand as filming went on. He describes the last day of shooting as "unlike anything I experienced in my life, and I never ever want to experience that again."