Trivia: The song that is played to wake everyone up is Edith Piaf's "Non, je ne regrette rien". Marion Cottillard (Mal) played Piaf in the biopic La Vie En Rose and won an Oscar for it.
Trivia: Joseph Gordon-Levitt performed all his own stunts during the fight scene in the spinning hallway.
Trivia: When Cobb asks Ariadne to draw a maze, she draws several that are "too easy." She then draws a circular maze which Cobb deems difficult enough. However, this maze is actually impossible to solve. If you pause and try, you will see it is just a dead end. [Technically a mistake, but the movie needs to be paused, so doesn't really count. Worth a look regardless.]. (00:25:40)
Trivia: Christopher Nolan's cousin Miranda has a cameo as an air hostess.
Trivia: Elliot Page, who was known as Ellen Page at the time of making this film, suffered a shingles outbreak caused from the stress of making the film and was worried about being replaced by Keira Knightley. Page also suffered a panic attack at the film's Paris premiere.
Chosen answer: The details are never explained in the depth, but all the dreamers are connected via the briefcase, so it seems likely that the briefcase acts in some way as a networking hub for them to share the dreamspace.
Tailkinker ★