For the first movie, the characters were sculptured in clay by the artist Kent Melton, who had done similar work on Disney characters for “The Lion King,” “Aladdin” and more. Back then, animators were not quite able to replicate the fine detail of Mr. Melton’s work in their computer models. “On this film,” Mr. Bird said, “we went back to the original sculpts and were able to get the characters exactly the way they were designed.”

One of the trickier characters was the brooding teenage daughter Violet. Mr. Bird had written her as insecure and hiding behind her long hair. But back then hair was very difficult to render. It took the animators six months of testing and tweaking, and the hair still didn’t look right. “They started asking me, ‘Does she need to have long hair?’” Mr. Bird said. “And I said, yes! That’s her character arc!” They got her hair working correctly at the last minute.

In the sequel, Violet’s hair is flowing wonderfully, but there’s much more to it, and the characters in general. “It’s not just the way they look in a freeze frame,” Mr. Bird said. “It’s the way they are able to handle expression changes.” The latest software lets the filmmakers build elaborate digital models, and can make skin move and respond in subtle ways, like a human’s would. “Then the animators test-drive these models the way a racecar driver would test-drive a Ferrari,” Mr. Bird said.

A Bigger (Much Bigger) House