In “The Great Lillian Hall,” Jessica Lange plays a veteran theater actress — a legend of the Broadway stage — who is always putting on airs, reciting bits from her favorite roles, and carrying on in the tradition of fabled actresses who get known for playing characters like Blanche DuBois because they’ve actually got a lot of Blanche in them. (They believe their own illusions.) Yet just because Lillian Hall is a flamboyant grand dame doesn’t mean that she’s not showing you who she is. Lange, a beauty at 75, has a face that has only grown more expressive with the years. In “The Great Lillian Hall,” that face is a map of emotion we read. Even when Lillian is being deceptive (even when she’s deceiving herself), the majesty of her feelings shines through.

There’s a moving scene in which Lillian is seated on a porch with her adult daughter, Margaret (Lily Rabe), who she never had time for when she was raising her; she was always acting, doing eight performances a week. At night, though, she would come home in time to sing the young Margaret to sleep, and now, on the porch, she gently sings that same song — “Hush little darling don’t you cry…” Her voice is old now, and it cracks, and what we see and hear in Jessica Lange, expressed in emotions as delicate as parchment, are three levels of awareness: an ache of nostalgia; the regret Lillian now feels over what an absent mother she was; and something new — a quiet chasm of sadness about the fact that she’s now going away, to a place from which she’ll never return. For what no one else knows is that she’s been diagnosed with dementia.

There have now been a fair number of movie dramas that deal with dementia, and I’ve been on the record as sometimes finding them touching yet dramatically frustrating. As the main character recedes, there’s a way that he or she can also recede from the audience. “The Great Lillian Hall” solves that problem in a simple way. The film takes place during the early onset of Lillian’s symptoms, so that even though she’s in rehearsal for a major new Broadway production of Chekhov’s “The Cherry Orchard,” where she has to contend with memory issues, the movie isn’t some gothic medical soap opera in which she suddenly starts to forget who she is. Rather, it’s about how Lillian, saddled with this devastating diagnosis, makes a peace with where she’s going by taking stock of who she’s been.

Her symptoms do cause some drama in the rehearsal process. She flubs her lines, screws up the blocking, forgets what act she’s in, and at one point literally falls on her face. Her most dramatic symptom, however, remains offstage: She keeps hallucinating that she’s seeing her beloved late husband, Carson (Michael Rose), a theater director who for some reason looks like an elegant European drug trafficker. David (Jesse Williams), the director of “The Cherry Orchard,” is a downtown star making his move to Broadway, and he hasn’t lost his faith in Lillian. But his tough-nut producer (Cindy Hogan) has. She keeps talking about bringing in the understudy to replace her.

The movie, written by Elisabeth Seldes Annacone and directed by Michael Cristofer, is a contraption that (mostly) works. It’s stitched together out of devices, like having Lillian’s neighbor, whom she flirts with on their stately adjoining Central Park South balconies, be a cornball Lothario played with jaded affection by Pierce Brosnan, or Lillian’s daughter saying a line like, “You never really wanted to be my mother. You just wanted to play the part!,” or the black-and-white faux-documentary-interview snippets that play like Bob Fosse Gone Cable Lite. The whole suspense about whether Lillian will make it through the rehearsal process and succeed on opening night — she’s the play’s box-office draw — carries you along, even as you realize it’s built around a major tinge of unreality. Is someone who’s struggling the way Lillian is really going to be able to perform this show all week long, for months on end?  

Yet Lange’s performance is so good that she gives this therapy-corn version of The Show Must Go On a worldly center you can roll with and almost believe in. Lillian relies on her veteran assistant, Edith (Kathy Bates), for just about everything, and these two actors have a cruelly intimate and feisty interplay you could listen to for hours. There are a couple of scenes that tap into the agony of dementia (and Lange, at those moments, is powerful), but “The Great Lillian Hall” is mostly a feel-good movie about using acting to turn the lemons life hands you into a grand illusion of lemonade.