Thomas Bezucha put the nine cast members playing the Stones through several weeks of rehearsal so they would bond well enough off-camera to convincingly portray a family. Since the script called for eight characters to use American Sign Language (ASL) to communicate with or interpret for Thaddeus, early rehearsals included a crash course in ASL. The cast members and some critics pointed out that their ASL use was sub-par, but it was a realistic portrayal of a hearing family's use of ASL, which is often perfunctory at best.
When Luke Wilson is telling Meredith about Everett's allergy, Sarah Jessica Parker shouted "I DIDN'T KNOW" in one take as a joke. You can tell as Luke Wilson genuinely laughs.
During the film's Christmas Eve montage, as Thaddeus and Patrick stroll hand-in-hand toward the inn, Patrick signs in American Sign Language that the evening sky looks beautiful, to which Thaddeus responds "just like you."
Composer Michael Giacchino re-recorded the instrumental track of "Have Yourself a Merry Little Christmas" as it appeared in Cita en St. Louis (1944), duplicating every note and matching every tempo of the original orchestration, which he then programmed simultaneously to enhance Judy Garland's vocal. Both tracks appear on the film's soundtrack album, allowing the listener to appreciate how closely the new recording matches the vintage one.
Playing the glue that holds the family together, Diane Keaton was the first actor approached to star in the film. Keaton has stated that she was instantly drawn to her role, as the many layers to Sybil's personality allowed her "to explore so many, often conflicting, emotions".