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."
Paul Conroy (Ryan Reynolds) is leaving a message for his family after all hope seems to be lost. He gives his social security number and birthday as 3/23/76, other then the month the day and year are correct. His real birthday is 10/23/76.
The movie never leaves the coffin Paul occasionally receives images and videos from his captors from unknown locations, he also hallucinates seeing the coffin lid opening at one pointb though nothing is shown but a bright light, there are also no flashbacks or cutaways showing anything other than what's happening inside the coffin he's in.
The government agent that is attempting to save Paul Conroy (Ryan Reynolds) is asked how many people have made it out of this situation alive. The agent states that there were two others who made it home alive, one being a man named Mark White. But when the agent thinks he and his team are about to save Conroy he states that they have actually opened the coffin containing a dead Mark White. This makes the agent's story a lie and hints that no one has actually been successfully rescued.
Assuming the casket is an average size, Ryan Reynolds occupies 85.33 liters, each breath is .5 liters, he breathes all of the air in the casket, and starting from his first audible breath of the movie to the moment he is buried in sand, he takes approximately 18 breaths a minute.