The audience was in a frenzy by the time actor-writer-composer Jason Segel took the stage at the First Parish Church on Sept. 12. He was there to present his kids’ book “Nightmares!” in conjunction with the Harvard Bookstore and 826 Boston, a creative writing nonprofit organization. Overheard mumblings included questions about Segel’s ability to “keep it PG” for the all-ages crowd, whether or not he would sincerely answer moderator and 826 Boston Program Coordinator Karen Sama’s questions, and how open he would be to discussing “Forgetting Sarah Marshall,” “Freaks and Geeks,” “The Muppets,” and the other projects that have made him a household name. From the moment Segel emerged—sweatered, svelte, and goateed—he was remarkably without pretense. Over the next 90 minutes, he effortlessly juggled an inspiring story of his creative inspirations, an adorable kid-participation interlude, and a prolonged and startlingly open Q&A. Any doubts about Segel’s willingness to fully commit to the event were emphatically put to rest.

Sama and Segel began by discussing Segel’s history with writing. After “Freaks and Geeks” was cancelled in 2000, the 21-year-old Segel found himself out of work. “I was in this weird period where I was too old to play a kid and too young to play a doctor or a lawyer and nobody was casting me,” he said. “I just thought, ‘I’m going to have to live with my parents forever.’” He appealed to his mentor, writer and director Judd Apatow, who had been the creative force behind “Freaks and Geeks,” and found a new direction. “He told me, ‘Jason, you’re kind of a weird dude. Look—here’s what you do now: you write.”

Segel’s first project was a screenplay version of what eventually became “Nightmares.” “I sort of got caught around this age of eight to 10 by movies like ‘Labyrinth’ and ‘Goonies’ and the Roald Dahl books…. They made me feel like I might find buried treasure or that there was still magic out there,” he said. “Every rational voice was telling me it’s impossible to become an actor, to write a movie, to sing songs in movies, to write a book, but I just, for some reason, believe it’s possible…that resolute belief is what it takes.”

The screenplay, Segel’s ode to the career-inspiring media of his youth, sat on a shelf for more than 10 years after he completed it. Then, after the success of “The Muppets,” Segel decided to try another project for kids and began morphing his screenplay into a book. Written with Kirsten Miller, creator of the popular kids mystery series “Kiki Strike,” it is the first book in a planned trilogy about young Charlie Laird, who suffers from horrible night terrors and has moved into a creepy purple house after the death of his mother. The perpetually exhausted Charlie is forced to spring into action after his nightmares begin intermingling with his waking life. Segel explained his decision to pursue children’s literature in particular. “Kids’ imaginations are so much better than what you could put on screen,” he said. “A kid gets to picture these nightmares however she wants.”

Segel didn’t wait long before testing out his thesis about the youthful imagination. He asked for kids in the crowd to tell him their scariest dreams, which led to a number of convoluted and hilarious stories from the under-10 demographic about purple aliens, vats of goo, and close encounters with fish people. Segel wasn’t finished—he next invited kids up to read the first chapter of the book. Three brave souls, all around nine or 10, came to the stage and delivered impressive orations of the initial descriptions of Charlie, his house, and his bad dreams. Midway through the readings, a six- or seven-year-old boy named Luca skipped down the center aisle, asked to read, and proceeded to deliver a spotless, book-on-tape level performance of the end of the chapter. Segel’s mouth was appropriately agape.

The theatricality only increased during the Q&A portion of the event. A nervous fan asked Segel what his favorite show tune was and, after Segel decisively answered with “The Confrontation” from “Les Misérables,” asked him to sing it with her. Rising to the challenge, Segel belted out the first lines (“Valjean, at last we see each other plain”), and the duet improbably began. Segel’s spontaneity shone throughout the event; despite his celebrity, he embraced fully every chance to interact with his fans and seemingly reveled in the intimacy of the event.