Most of the reviews assume that there's no supernatural element in the story, and that Henry's experience is simply an extended hallucination. I'd like to suggest a different reading.

After regaining his sight, Henry's father says, "The Buddhists got it right. It's all an illusion." This could be taken as a reference to the Buddhist idea of Maya, but it could also be understood as a reference to the immediate postmortem state described in the Tibetan (Buddhist) Book of the Dead. In this state, we are told, the newly dead (or not quite dead) person inhabits a world of thought-forms. Unless he recognizes these forms as illusory, he will be trapped on the wheel of rebirth.

It's clear that the random persons gathered at the scene of Henry's accident are the inspiration for the thought-forms peopling his experience. But I'm not so sure the same applies to the three people Henry knew in life - mother, father, girlfriend. All of them died before Henry, and I suspect that when they appear in his story, they are not thought-forms but spirits sharing his dream in the bardo.

Each has a different level of awareness. Henry's father is entirely unaware. Like Bruce Willis in The Sixth Sense, he does not know he is dead. He does not even know his own identity. He is blind, literally and symbolically, until Henry opens his eyes, prompting him to say, "I see ... everything," as well as his words about Buddhism. Having seen through the illusion, he walks out of the shared dream experience and disappears.

Henry's mother is somewhat aware all along. She knows who she is, she knows something bad happened, but she is confused about the details and does not know she is dead. Her cloudy thinking is indicated by the scarf around her head, suggestive of brain trauma. Like an earthbound spirit, she haunts her own house (which she visualizes unfurnished, as it will be after the estate sale) and conjures up a long-dead pet for company. She recognizes Sam as Henry because all the thought-forms are projections of Henry's mind.

Finally, Henry's girlfriend appears to be fully aware of her circumstances. She is shown acting a role in a play, which is precisely what she is doing in Henry's experience. She places special emphasis on Hamlet's line, "I could be bounded in a nutshell and count myself a king of infinite space, were it not that I have bad dreams," an exact description of Henry's condition (spoken in Sam's presence, because, like the mother, she recognizes Sam as Henry's alter ego).

Her name is given only as Athena. I suspect this is an alias generated by Henry's mind; Athena was a goddess, and Henry was devoted to this woman. Even Henry's last name, Letham, appears to be an alias. It's an anagram of Hamlet, whose bad dreams disturbed the infinite space of his subjective prison, and who is the literary character most closely associated with pondering one's own mortality.

There are many possible interpretations, of course, but I think this one may come closest to the filmmakers' intentions. Then again, I might be completely wrong!