And that song he has composed is in perfect, two-part harmony: one provided for Reiser as the babbling boyish Ben Kleinman; the other for Peter Falk, as the impenetrable Sam, whose wife leaving him late in life leaves him changing his tune from Pop to singing the blues.
The thing about "The Thing About My Folks" - yes, Mom (Olympia Dukakis) gets screen time, too - is that the song is from the heart and the humor from Reiser's rat-a-tat mind.
The film opens in the area on Friday, Sept. 16.
And while there are three sisters involved - Chekhov, check out these disparate housewives - it is the son, Ben - bent over from carrying his parents' marriage on his shoulders so late in the game - who is the rematch maker of Hysteria Lane.
If only there were some magical powder that could be sprinkled on their tsuris - beyond the omnipresent talcum powder Sam slaps on post-shower - maybe this family tree could be saved from sawing its own limbs off.
Miracle-Gro for grown-ups? It's harder to save a brood when the brooding father's more cactus than carnation, thinking all his efforts to assure his family's welfare should fare better in their hearts than it does.
How close to hearth and home does the character play?
"He's 50 percent Peter Falk, 50 percent my own father," says Reiser lovingly of his late Dad, whose admiration for Falk in "The Cheap Detective" made Paul realize he didn't need a private eye to uncover the ideal actor to play him.
"Peter Falk and my father are very much the same," says Reiser.
What remains the same is the filter of time: "As you get older," says Reiser, 47, himself father of two - he just celebrated his 17th anniversary with wife Paula - "you realize your parents don't look so dumb - and that you're not as smart as you thought you were."
"Folks" is a smart move and movie for Reiser, who's attuned to the parents' problems and guilt pangs, and has taken all their cries and whispers and shoved them into a Baby Boomerbox.
As for how audiences tune into this topical comedy, with a tam for trauma, too: "My mother, who is completely objective, says it's the finest film ever made," Reiser deadpans.
Audiences have long been mad about the boy, too; Reiser and Helen Hunt chased down the pot at the end of the rainbow in their long-running series "Mad About You" (1992-99), which the comedian based on the relationship he shared with his psychologist wife.
And if family life has a way of finding its way into his TV and film fare, well, it's all relative when it comes to assuaging the occasional worry. Reiser's mother - who shared a wonderful life with Paul's father, her husband - was concerned that audiences would think she - like Dukakis' character - left her husband.
"They'll think it's true," Reiser recalls her worrying.
"No," I told her, "they'll know it's all made up. It's about other parents."
True, but Not-So-True
Indeed, concedes the actor with his delightfully trademark boychick smile, "it's all a work of fiction. However," he says devilishly, deliciously, "it's based on actual Jews."
Actually, Reiser's as Jewish as they come. The native New Yorker - who once refused to openly identify his "Mad About You" characters as Jewish; of course, it was difficult to conceal when pell-mell Mel Brooks played his mad-libbing uncle in the show - leaves no doubt that the Kleinmans are landsmen.
And if he didn't author the play "Conversations With My Father" - Herb Gardner did - Reiser at least understands that a heart-to-heart talk can be the sole music a son ever needs.
Which is why this "film is about conversations I had with my father - or felt I had with him."
And feelings is what the film is all about. "It's really about this fantasy I had about having more time with Dad, time to get to know each other."
Not that there wasn't a chance. After graduating from college, Reiser rose to his dad's challenge and went into his family's wholesale health-food business, in charge of the Oklahoma region.
Oh, what a beautiful mourning: Reiser, enamored early on of comedy even while majoring in music at State University of New York/Binghamton, tussled with himself in Tulsa before going against the family grain and finding the New York scene more okay than OK.
From granola bars to comedy bars: "It was an emotional scene, but my parents were supportive."
Ya see, the thing about his folks - and Reiser's own three sisters - was that love never meant having to give him tsuris.
Before long, Reiser's wry humor was attracting crowds, and he was making a healthy career away from the health-food business. As for dad, what unbleached flour could compete with a son making comedy his bread and butter after a powerful debut on "The Tonight Show"?
Organic, schmorganic! Papa's got a brand-new bag?
No - the son does!
Paul's father watched and exclaimed at the TV, "Look, he's on Carson!"
Since, there's been even more television; clubs; films ("Diner," "Beverly Hills Cop," "Aliens"); telefilms for Showtime; and development deals put together by his own company, Nuance Productions for NBC and Showtime.
And what does Reiser have to show for time spent as a happy husband and father of two? Two best-selling books with his novel take on both: Couplehood and Babyhood, which he wrote after son Ezra was born in 1995.
And, now, with this movie, he's in the father 'hood. What could tear at the tear ducts more readily than the film's simplest - and simply wonderful scene - in which son and father fall asleep on an autumnal bed of leaves and enter dreamland with their emotional sandbags depleted? Reiser smiles at the memory of making that scene: "Just two Jews lying down talking about their loves and hopes."
And the former college music major - who, as a teen Hebrew-school student, wrote the musical "The Beatles Meet Purim" - has the beat of boys and their dads down.
Now, the "Folk" composer hopes audiences will share his joy to the world for what it is. "It feels like a duet," he muses.
One in perfect two-part harmony.