There seem to be several of these. Some character – usually very important to the plot – is spoken about, referred to, and described so well (sometimes better than characters appearing onstage) that they are clearly major features of the play/movie. Yet they never actually appear.
Marguerite (Andrew Wyke’s wife) in Sleuth is the first one I think of in this context. She’s having sex with both Wyke and Milo Tindle, and is clearly one of the drivers of plot motivation. In the 1972 film they even felt they had to include a life sized painting of her. Yet she never appears onstage in play or either film.
In the play and film Same Time Next Year we never see either George’s wife, Helen or Doris’ husband, but both are virtually characters in the story.
Alex Marshall is the dead guy , for whose funeral the rest of the characters in The Big Chill assemble, but you never see him, even in flashback. They even filmed scenes of Kevin Costner as Alex (as a dead body), but they never made it into the finished film.
Neil Simon’s first play, Come Blow Your Horn has constant references to Aunt Bessie (I might be misremembering the name) throughout. She’s not really important, but it’s a throwaway gag at the end when she suddenly shows up, played by a real actress. Simon invokes a lot of never-seen characters by name in the play, but none as much as this much-referred-to Aunt (Snow Eskanazi, his first use of the name “Felix Unger”).
In Big Night the major dining event is being made in expectation of the arrival of famous singer Louis Prima, who never shows up. (Well, he DID die almost twenty years before the movie was made, but they could’ve had someone portray him)
The Godfather II. Don Corleone (the Marlon Brando version) never appears but he looms large over the entire story in flashbacks to his youth. In the final scene he shows up in the next room, but is not shown.
I saw “Cat on a Hot Tin Roof” (Elizabeth Taylor/Paul Newman film version of the play) recently. A lot of the plot directly or indirectly revolves around the longing the Newman character feels for his deceased male “friend” (in the play, it’s clearer they were lovers).
I love that movie to death, but the final scene is one of the most crazily misjudged endings imaginable. It turns a melancholy tragedy into a bad sitcom and turns Vito Corleone into Vera Peterson.
The eponymous Amy in Chasing Amy never appears onscreen. The main woman character is Alyssa; Amy is a character in a life lesson monologue delivered by Silent Bob in a rare bit of verbosity.
You get glimpses of her in clips at the very beginning, but they aren’t really necessary. It’s her death that drives the plot.
There’s also Lucy in the 1985 film Creator. She’s the deceased wife of Nobel Laureate Harry Wolper (Peter O’Toole), and he’s trying yo clone her from some of her preserved cells. (The movie is a LOT better than this capsule description suggests). Aside from her appearance as cells and the beginnings of a clone, Lucy only appears in a “vision” briefly at the end, played by Karen Kopins. There was also a scene with her and a younger Harry Wolper riding a roller coaster at the seashore, but it goy cut from the film, and only showed up in some TV broadcasts.