Originally planned to be filmed in English language. Once Guillermo del Toro became involved as a producer and Universal Pictures saw an opportunity to start its first Spanish production, the idea was changed.
This is the second time that Belén Rueda plays a character named Julia, who coincidentally also suffers from a degenerative disease, after The Sea Inside (2004).
Julia Gutiérrez Caba and Belén Rueda have previously worked together in Los Serrano (2003), in which they played mother and daughter respectively.
The score was created by Barcelona-based film composer Fernando Velázquez.
Belén Rueda also played the lead in another film produced by Guillermo del Toro, The Orphanage (2007).