Who doesn’t love Diane Keaton? Or frankly, want to be Diane Keaton?

The Oscar-winning star has had a film and television career spanning six decades, she’s a fashion icon, and she’s done it all in her own singularly unique and quirky way. It’s not surprising then, that in the fantastical and fluffy comedy “Mack & Rita,” a struggling young writer wishes to be as cool and confident as Keaton herself, or someone like her, as in, older.

Rendered literal, that wish results in a tale that could be described as “Freaky Friday” meets “Old.” It’s a cute concept, but one that turns out to be a lemon once you start kicking the tires.

Watching Keaton read the phone book would be entertaining.

Unfortunately, the phone book would have made more sense than the screenplay for “Mack & Rita,” which ditches character establishment and a clear conflict for fish-out-of-water physical comedy and some vaguely affirmative lessons about learning to be yourself, unapologetically.

Twenty-something Mack (Elizabeth Lail) is an author turned social media writer/influencer. Though she looks young and hip, she’s truly an old soul who dreams of living like her dear grandmother, swanning about in colorful caftans, not caring about what other people think. This desire for the caftan life is apparently a struggle for Mack, as she violently resists the youthful capers of her friends during a Palm Springs bachelorette party for her best friend.

Worn out from a bottomless brunch, Mack stumbles into a “past-life regression pop-up” and clambers into an old tanning bed at the behest of Luka (Simon Rex). He guides her through a meditation about who she really wants to be, and out pops Diane Keaton, naturally. Mack is suddenly the bold and stylish 70-something she’s always dreamed of becoming one day.

Posing as an “Aunt Rita” until the problem can be remedied, the older Mack slides back into her life with a few bumps along the way. But she’s also got a new groove as Rita, flirting with her next-door neighbor Jack (Dustin Milligan), and becoming a surprise Instagram sensation.

Any and all age swap shenanigans, baffling scenarios and flaws in the concept could be forgiven if we better understood Mack, whose issues seem muddled and trivial. Rita, well, who even is Rita? She’s supposed to be Older Mack but she’s just Diane Keaton, who does her signature adorably neurotic routine (if it ain’t broke). However, there’s no consistency of character or performance from Lail to Keaton and back again, and it always feels like Mack AND Rita, not that they are the same person.