It’s hard to even contemplate how difficult it must be to write a song like “That Thing You Do!” Tom Hanks directed That Thing You Do! in 1996, when he was at the absolute peak of his movie-star powers. This was Hanks’ “one for me,” his passion project. He wrote a cinematic love letter to the rock ‘n’ roll bands of the mid-’60s — the one-hit wonders who came surging in out of nowhere, briefly captured the hearts of teenage America, and then disappeared forever. To pull this off, Hanks needed a ridiculously catchy song that sounded like 1964. This is not an easy task to manage. If it were, a whole lot more people would still be writing 1964 bangers.

The song that drove That Thing You Do! needed to sound effortless, which was part of the difficulty. It had to be simple and spiky and joyous and maybe a little bit amateurish. It had to be something that a few teenagers in a go-nowhere Pennsylvania town could’ve believably come up with. It also needed to be a song that could’ve plausibly soared up the charts in one of the most competitive moments in pop-music history. And it needed to be strong enough that people watching the movie wouldn’t get sick of the song, even though they heard it again and again throughout the film. The song had to be exciting every single time it played.

In 1996, Adam Schlesinger, then in his late twenties, had just signed his first publishing deal. His band Ivy, a slinky and vaguely sophisticated indie-pop group from New York, had released one album. Schlesinger had formed the power-pop group Fountains Of Wayne with his old college friend Chris Collingwood, and they’d made a demo that had gotten them signed to Atlantic, but they hadn’t yet released their debut album. Upon hearing about this film project, Schlesinger wrote “That Thing You Do!,” recording the demo with his friend Mike Viola of the Candy Butchers. Other songwriters submitted songs for the movie, too, but Tom Hanks was smart enough to figure out that Schlesinger’s song was the one.

“That Thing You Do!” succeeded better than anyone could’ve hoped. The song absolutely propelled the movie; it’s the main reason the film is as rewatchable as it is. It’s also almost uncannily believable as a 1964 chart hit. I’ve talked to people who didn’t realize That Thing You Do! was fiction — who thought it was a biopic about a real band who’d made a real song. That doesn’t happen unless the song absolutely nails its aesthetic, unless it works on every level.

The song became a legitimate hit in the summer of 1996, peaking at #41 on the Hot 100 in a time that wasn’t exactly hospitable to peppily nostalgic guitar jams that were not the theme song from Friends. Schlesinger was nominated for a Best Original Song Oscar, and he lost it to “You Must Love Me,” an Andrew Lloyd Webber/Tim Rice song that Madonna sang in Evita. “You Must Love Me” was basically guaranteed to win that Oscar; it was a superstar showstopper forcefully jammed into a big musical for the express purpose of winning that award. It’s nowhere near as good a song as “That Thing You Do!,” and it’s nowhere near as important to the success of its movie. Schlesinger should’ve won, but the mere fact that “That Thing You Do!” exists — that it’s able to propel the film the way it does — is some kind of miracle.

Schlesinger specialized in that kind of miracle. With Fountains Of Wayne, Schlesinger and Chris Collingwood wrote dozens of brilliant, glimmering bops about regular people, schmucks living drab and unremarkable suburban lives. Most of the best Fountains Of Wayne songs work as songwriting exercises, attempts to capture the people who don’t often have pop songs written about them. “Denise”: A love song to a Lexus-driving Queens travel agent with “a heart made of gravel.” “Comedienne”: A lament for a struggling stand-up who might never get to quit her day job. “Leave The Biker”: A lost plea to the girlfriend of a cartoonish lout. “All Kinds Of Time”: An ode to a high-school quarterback getting to experience one moment of magical certainty. “Prom Theme”: A ballad dedicated to an ephemeral night that two people get to have while admitting that, once it’s over, “we’ll work until we die.”

“I tend to write songs that are about something pretty specific,” Schlesinger once wrote in The New York Times, and this was a massive understatment. If you were feeling uncharitable, you could see something cold and calculating in what Fountains Of Wayne did. They used the universal language of pop music to write tiny, clinical short stories about the characters they’d sketched out in a few precise strokes. But if you were willing to meet Fountains Of Wayne where they were, then you realized the tremendous grace and empathy that they brought to what they did. They gave themselves songwriting assignments, and they used those assignments to make something moving. That’s what pop music can do.

This even holds true for “Stacy’s Mom,” the Schlesinger-written song that briefly transformed Fountains Of Wayne from theoretical pop musicians into actual ones. “Stacy’s Mom” is big and dumb and hooky and simple, but it’s just as much a character piece as any of Schlesinger’s other songs. The narrator is a hopelessly horny teenager with a deluded dream in his heart: “I know that you think it’s just a fantasy/ But since your dad walked out, your mom could use a guy like me.” On the pop charts, “Stacy’s Mom” did better than “That Thing You Do!,” peaking at #21. It was a rare and unlikely feat for a group of nondescript late-thirties power-pop professionals, managing to compete in the Usher era.

In that Times piece, Schlesinger wrote, “I think I initially started inventing characters in my songs because I didn’t want to write directly about myself.” But after “That Thing You Do!,” he got extremely busy writing songs to service characters that he hadn’t made up. Schlesinger’s list of IMDB credits is long and wild. If someone needed a pop song that was catchy and more clever than it had any right to be, Schlesinger was right there, working on Josie And The Pussycats or Ice Age: Continental Drift or the Disney Channel musical Descendents. And as this Annie Zaleski piece shows, he did something similar for plenty of fellow musicians, from Bowling For Soup to the Monkees.

As a songwriter, Schlesinger was never even the tiniest bit precious. He was a monster hooksmith, capable of cranking out jams in bulk to suit just about any conceivable dramatic situation. So Schlesinger’s real masterpiece might not have anything to do with Fountains Of Wayne. It might be the four seasons that Schlesinger spent working on Crazy Ex-Girlfriend, the sneaky-great CW musical series about a woman who makes an ill-advised move across the country to chase an old high-school flame. Working on that series, Schlesinger, working with star/creator Rachel Bloom and various other writers, came up with more than 150 songs. A whole lot of those songs are just straight-up incredible.

Working on Crazy Ex-Girlfriend, Schlesinger came up with pastiches of dozens of genres: Club-rap, middle-of-the-road ’70s easy listening, techno-pop, melisma-drunk R&B, hair metal, torch balladry, virtually every permutation of musical theater that you could imagine. He produced those tracks, too. The whole body of work is a wild and herculean undertaking for virtually everybody involved in Crazy Ex-Girlfriend, but maybe especially for Schlesinger. The Crazy Ex-Girlfriend songs are schticky and clever and jokey, but they also get into the loves and longings and hopeless depressions of a set of characters in an unremarkable California town. In that, Schlesinger’s work on Crazy Ex-Girlfriend is almost an extension of what he was doing on Utopia Parkway — just a wildly more expansive and ambitious version of it.

The show is endlessly watchable precisely because of those songs, and because of the expectation that one of these songs can suddenly rear up whenever you’ve forgotten to expect them. Maybe Schlesinger wasn’t doing the mystical personal work that we expect songwriters to do when he was writing all of those things. But the man was working. He was cranking out material at a high level every single day. Those of us paid to do the same at our own professions — those of us who, let’s say, are paid to blog relentlessly five days a week — should regard Adam Schlesinger as a hero, and as a monumental loss.

As it happens, Adam Schlesinger’s biggest hit, at least in pop-charts terms, isn’t “That Thing You Do!” or “Stacy’s Mom” or anything he did for any movie or TV show. It’s “Just The Girl,” a 2005 song that Schlesinger wrote for the Click Five, a mop-topped boy band comprised mostly of Berklee College Of Music students. “Just The Girl” is a processed, professional, infectious teen-longing jam that evoked the Cars and Ashlee Simpson at the same time. It peaked at #11, and the Click Five never made another hit after it. Somebody should write a movie about that. The song already exists.