Twins Spencer and Bradley Pickren were cast together to play Little Jack, a common industry practice that let producers work within child labor time limits and swap in a fresh sibling if one became tired or upset.

The twins' mother was a pediatric nurse who had taught them sign language from birth, though they had to learn a few additional signs specifically for the movie.

The Pickren twins performed well on set most of the time but were never at ease with Ben Stiller after filming the head-butting sequence.

Although the head-butt itself was achieved with a digitized effect, Ben Stiller had to sell the moment by grabbing his face, cursing loudly, and smearing fake blood under his nose, which upset the young twins.

To help build the character of Roz Focker, a sex therapist, the crew consulted real sex experts Jennifer and Laura Berman.

The Bermans' advice about professionals in their field needing to avoid exposing their own children to too much sexual knowledge too soon was worked directly into the film.

The film broke box-office records previously held by The Lord of the Rings: The Return of the King for both the highest Christmas Day gross and the biggest Tuesday gross.

It later surpassed records held by Cast Away and The Lord of the Rings: The Return of the King for New Year's Eve and New Year's Day grosses.