Ann Miller, who plays Essie, was only 15 when the film was made, even though she's playing a married woman. The ballet positions she had to do were so painful they made her cry—so Jimmy Stewart brought her candy on set. Because Jimmy Stewart is awesome. (Source)

Lionel Barrymore, who plays Grandpa, was really on crutches; he had severe arthritis, which would eventually force him into a wheelchair. (Source)

The Production Code which reviewed the film was worried that Southern audiences would be offended by the black characters, because they seemed to interact too familiarly with the white family. They were servants, but not servant-y enough for white racists. Ugh. People are horrible. (Source)

The play You Can't Take It With You had nineteen acting roles. The film version has a hundred and fifty three. (Source)

Capra identified strongly with Tony, according to his niece. In fact, Capra, like Tony, had hoped to develop solar energy while he was in school. (Source)