There’s a tradition of actors penning a good script as a vehicle for themselves to star in, write, and direct, and of overcoming the star’s lack of box-office prowess by filling the other main roles with known quantities. “Fantasy Life” is another such entry. It’s pretty good – slight but pleasing, a sort of romantic comedy about smart, neurotic New Yorkers.

The auteur here is Matthew Shear, who’s been acting in major films and TV programs since his mid-twenties (including multiple stints in some cringe comedies and a lead role in “The Alienist”). His character is Sam Stein, a paralegal who learns he’s being laid off, spirals into a series of panic attacks, and worries that his latest, biggest failure is proof that the ego-battering anxiety he’s dealt with throughout his life is an accurate prediction of his future, too.

He tells his troubles to his psychiatrist, Fred (Judd Hirsch, who got one of his two Oscar nominations for playing the therapist in “Ordinary People,” and could’ve worn one of his fuzzy sweaters from that movie as an in-joke). To Sam’s astonishment, he is hired in Fred’s waiting room right after the session by Fred’s wife, Helen, portrayed by another showbiz legend, Andrea Martin. Their son, David, is a longtime wannabe rock star who just booked a plum gig that very night and needs a babysitter to watch his three daughters. David is played by Alessandro Nivola, a seasoned character actor and sometime leading man who fits comfortably in every genre.

It takes a certain amount of chutzpah to think you can cast yourself against these formidable actors and not get wiped off the screen like a bug that ran headfirst into a car’s windshield. But Shear is up to the task, channeling the panicked, self-abasing, borderline schlemiel energy perfected by the young Albert Brooks and carried forward by a succession of comedic leads up through the present day.

The babysitting assignment does not go well, but David likes Sam and keeps bringing him back. You might expect that this is going to become a movie about a loser whose self-esteem gets a boost when he becomes a surrogate or adjunct parent to somebody else’s kids. But “Fantasy Life” quickly elbows the girls aside to concentrate on Sam’s developing relationship with David’s wife Dianne, a former movie star with mental health issues whose chattering charisma sparks a deep crush.

It turns out that the movie is mainly interested in Dianne and Sam by way of Dianne. And why wouldn’t it be, when Dianne is played by Amanda Peet, who’s been steadily employed on TV but hasn’t done a movie in 10 years? Shear told Indiewire’s Kate Erbland that although he didn’t write the character for Peet specifically, it soon became clear that she embodied the character, and could be the name that would get the film financed. Some of the details of Dianne’s life were drawn from Peet’s own experience, including people coming up to her in public to excitedly compliment her on one of her performances, only to realise they’d mistaken her for Lake Bell.

As a budding triple threat, Shear is no Orson Welles, or even Bradley Cooper, but he’s a promising prospect. He has a knack for ping-pong banter and solid instincts for how to shift out of one mode (embarrassment-laced slapstick, for instance) and into another mode that seems as if it would not be compatible but pairs surprisingly well (like navel-gazing self-pity).

The major problems are structural and psychological. The movie covers a long timespan for a comedy of frustration, and its decisions about which important events in the characters’ lives to show us and which to keep offscreen and describe only verbally are often mystifying. This gives the movie a shuddering stop-and-start momentum that keeps thwarting our willingness to invest in the characters, who are already a bit vague because they don’t have much in the way of inner lives, and Shear is reluctant to offer details that could help fill them in any other way. But the top-to-bottom cast of proudly eccentric actors, including Holland Taylor, Jessica Harper, Zosia Mamet, and Bob Balaban (as Dianne’s father), ensures that every scene has moments of truth, and the filmmaker’s empathy pushes the movie over the finish line.