For Karolyn Grimes, this is the most Wonderful time of the year. The former child actress is booked solid each holiday season to introduce screenings of Frank Capra’s It’s a Wonderful Life. It is perhaps the most cherished, quoted, and oft-seen American Christmas film—and she co-stars as one of its most cherished and oft-quoted characters.

Grimes portrayed ZuZu, George Bailey’s youngest daughter: the “little ginger snap” with the petals, who utters the immortal line, “Look Daddy, teacher says every time a bell rings, an angel gets his wings.”

Hers is an odd sort of celebrity. Her name is most likely unfamiliar; she is not often recognized when she goes out. “But when they find out who I am, believe me, they light up,” Grimes tells Vanity Fair. “That to me is a real blessing.”

This year marks It’s a Wonderful Life’s 70th anniversary, a momentous occasion for a film whose life hasn’t always been so wonderful. Though the movie initially received good reviews, it underperformed at the box office. “This was the first full Christmas after the war,” Jeanine Basinger, author of The It’s a Wonderful Life Book, noted in an interview. “Everyone was celebrating and in a happy mood. The film has an upbeat ending, but you suffer quite a bit before you get there. And that was part of its problem.”

It’s a Wonderful Life was nominated for five Oscars, including best picture and best actor, but it was shut out at the Academy Awards. (The top honors instead went to The Best Years of Our Lives.) Its standing eventually fell so low, in fact, that in the 1970s, the film’s neglected copyright was allowed to lapse, and it fell into the public domain. This turned out to be a blessing—television-station managers across the country had been given a Frank Capra movie that they could show for free during the holidays.

And they showed the heck out of it. “I remember one Christmas Eve when it was in the public domain, my wife and I played TV roulette with it,” recalled Leonard Maltin, film historian and contributor to the It’s a Wonderful Life Book. “We literally kept changing channels and came upon it in different stage of its progress. And you can’t not watch. You can’t turn it off.”

Mr. Gower, played by H.B. Warner, and a young George Bailey, played by Bobbie Anderson, in the famous slap scene. From Everett Collection.

That’s because the film, about a suicidal man whose guardian angel shows him what life would have been like had he never been born, is brimming with classic lines and indelible moments—so many that you when ask people to name their favorite, you’ll seldom hear the same answer twice. “The scene that tears me up every time is when (pharmacist) Old Man Gower slaps young George around, and then realizes George saved him from making a terrible mistake that could have killed someone,” Maltin offered.

For Mary Owen—daughter of Donna Reed, who portrayed George’s steadfast wife; and, no, she was not named after Mary Bailey—it’s the emotional telephone scene that seals George’s small-town fate. “It’s a phenomenal scene,” she said. “And it’s a great story, too. When it was time to do that scene, Jimmy Stewart was very nervous.” It’s a Wonderful Life was the first film he’d made since returning home from the war, and “it had been a long time since he had kissed a woman on-screen. They did the scene in the first take. When Capra said, ‘Cut,’ the script supervisor mentioned there was some dialogue missing . . . I’ve heard the censors thought it [was] too racy.” No matter; the moment was powerful enough without words.