A few weeks ago, I had the opportunity to see Jason Segel at a book tour stop to promote his 2nd book in the triology (first one here, including some of his motivations for writing the book https://www.producthunt.com/book...). Co-written with Kirsten Miller, Jason said it's quite rewarding to write in a partnership. He described their process as a very supportive one. Jason created the concept nearly 20 years ago, and had written the first book as a screenplay. Segel decided that kids are far more imaginative than adults, and also felt that reading is more rewarding than turning off your brain while watching a movie. He scrapped the idea of a movie and turned to Miller, who started to adapt the first book from the screenplay. Starting with an outline for the second book, both riffed off each others ideas to make a stronger book. Although I haven't watched the entire interview from BookCon, much of what he describes is the same. See here if you're interested in more of his story.
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