Contrary to so many reviews (including some of my own), Hollywood will never run out of ideas. Why? Because if all else fails, it can always write about itself, as it has for more than a century now.

The number of films and TV shows about Hollywood speaks to the insulated, self-referential nature of this thriving cultural pimple on the face, or some other part of the planet’s anatomy. Sometimes, Hollywood-themed films and TV shows are great — “The Bad and the Beautiful,” the Cukor “A Star Is Born,” TV’s “Episodes,” “Entourage” — and at other times, not so much. Remember Bette Midler’s sitcom, or was that the moment in 2000 when you scheduled yourself to blink?

“Doll & Em,” a faux reality comedy landing on HBO tonight, falls somewhere in the middle: It has some promise at first but quickly becomes predictable.

The “Em” of the title is co-creator Emily Mortimer, who plays herself, or, more accurately, a version of herself from the template for any show attempting comedy about real-life celebrities. Really: That’s the rule. Since the American public thinks all film and TV stars are probably insufferably self-indulgent freaks, regardless of their public personae, if you’re James Van Der Beek, Larry David, Matt LeBlanc or Emily Mortimer, you have to play yourself as at least very self-absorbed if not entirely monstrous.

The premise of “Doll & Em” is that Mortimer hires her real-life best friend, Dolly Wells, to be her personal assistant, and it’s a disaster. Despite Em’s assurances that the job is really just a way of helping Doll after a bad breakup with her boyfriend, almost at once, Em is expecting Doll to act as a real assistant, giving her specifics on how she likes her latte, including the size of the cup. For her part, Doll makes an initial attempt to be an assistant, but is quickly bitten by the attention bug and not only makes friends among the cast and crew of the film Em is making in Los Angeles but even gets herself hired as an extra and ends up stealing a scene in the film.

Once we get the pattern of how the women will act and react toward each other, the show becomes predictable but is rescued at strategic points by the presence of several guest stars playing themselves: Susan Sarandon, John Cusack and Chloë Sevigny.

Other characters are as vapid as we expect Hollywood types to be, bloated with their own self-importance and exhibiting a myopically skewed worldview. The details are often amusing, pointedly satirical and well written (by Wells, Mortimer and Azazel Jacobs, who also directs), but they end up feeling like ornaments on an old Christmas tree.

Sporadically entertaining though it is, “Doll & Em” could have been even more interesting if Mortimer had played herself as down to Earth and entirely free of the kind of craziness we expect from Hollywood stars. That kind of premise would have been more challenging to the show’s writers-creators, but it would have made it harder to fall into the predictability trap.