There are movies based on comic books, movies based on toys, and even movies based on amusement park rides, but nothing wears a bigger “hate me” sign on its back than a movie based on a novel written by a celebrity. To be fair, the absolute worst novels are written by politicians. From Senator James Webb’s bonkers military fiction to Jimmy Carter’s historical novels, in which characters introduce themselves to one another in briefing paragraphs (“My father is a Moravian minister, who teaches in our church’s boarding school. We live about two hundred yards from here, and I have seen you working in the garden. My grandparents used to live in Savannah, Georgia, and my grandmother lives with us now.”), books written by politicians are so uniformly bad that it makes their statesmanship look that much better.

But the very idea of celebrities who write novels is so offensive to the average reader that their books are doomed to failure before they even begin interviewing ghostwriters. If they write about the lives of the fabulously wealthy, they are fatuous and out of touch; if they write about the working classes, they’re patronizing. But the crime they commit most often is that their books are boring. William Shatner is one of the more successful celebrity authors, and his science fiction “Tek” series has only one saving grace: If you string all the titles together they sound silly (“Tek War,” “TekLords,” “TekLab,” “Tek Vengeance,” “Tek Secret,” “Tek Power,” “Tek Money,” “Tek Kill,” “Tek Net”).

So poor Ethan Hawke is triply doomed. Not only did he write and direct the screenplay for “The Hottest State,” which opens today, but he also wrote the novel on which it’s based. Published in 1996, “The Hottest State” was a New York coming-of-age book by a 26-year-old Mr. Hawke, who was still best known for his early roles as an easily bruised, post-adolescent stud in films such as “Dead Poets Society,” “Reality Bites,” and “White Fang.” Plot: William, a struggling, 20-year-old actor meets Sarah, a struggling 20-year-old singer, and they hook up. Then they go on a trip together, have a lot of sex, and break up. Eventually they get over each other. Then they turn 21.

William has abandonment issues because his dad left him, Sarah has commitment issues because her boyfriend cheated on her, and it’s all as predictable as airline safety instructions. The writing isn’t good enough or bad enough to be good (On Williamsburg: “There are many small shops with people standing out front smoking, drinking, and talking.”) It’s self-absorbed, self-important, and more than a little embarrassing.

The movie, on the other hand, is very good. Mark Webber plays William, and this time around the Tennessee Williams-quoting, cigarette-smoking pip-squeak isn’t taken straight; his proclivity to get drunk and break furniture feels like a kid trying act tough (and failing), rather than the actual tough-guy gesture it’s supposed to be in the book. Catalina Sandino Moreno plays Sarah with enough talent to show both sides of the coin: She’s as seductive as William claims she is, but you also realize that she’s not too far off the mark when she tells him that she’s shallow and not very interesting.

More important, Sarah’s mother has transformed from the book’s repressed, bitter WASP stereotype into Sonia Braga, star of numerous telenovelas, who turns her 10 minutes of screen time into a visit to a diva’s boudoir, her accent as moist and delicious as mashed potatoes. Late in the film, Laura Linney touches down as William’s mother and she lays waste to all the acting that came before her like a tornado in a trailer park. She and Mr. Hawke, playing William’s father, get less combined camera time than William’s hairy belly, but they prove the old Hollywood adage: Young people are only interesting on-screen when they’re half naked.

Mr. Hawke and Ms. Linney arrive fully equipped with matching sets of emotional baggage and, compared with the heartache lurking behind their ravaged faces, the romantic fantasies and petty tantrums of William and Sarah look like pretty thin soup. Fortunately, the two young actors do take their clothes off a lot, so it’s not a completely one-sided argument.

An anthem to young, stupid kids falling in love and growing up, “The Hottest State” is directed with the rueful perspective of a 36-year-old man who’s endured a broken marriage, a nearly-failed career, public humiliation, and become a father. Looking at Mr. Hawke’s photo on the jacket of “The Hottest State,” you see a callow, fresh-faced star desperate to be taken seriously. Looking at him playing his father in “The Hottest State,” you see a guy who’s been marinated in failure for a decade. And that’s why the movie succeeds where the book doesn’t. The novel was written by a celebrity. The movie is directed by a human being.