Trivia

The film was shot almost completely with hand-held cameras.

The title comes from the work of Dr. Duncan MacDougall of Haverhill, Massachusetts in the early twentieth century. MacDougall weighed dying subjects in an attempt to prove that the human soul was material and measurable. His experiment is widely regarded as flawed and unscientific.

The film was shot in chronological order (even though it's not in chronological order in its final cut).

Naomi Watts accepted the role without reading the script first.

At Sean Penn's urging, Naomi Watts hit him for real during an emotionally charged scene between them.