Ever go see a movie based on a book you really like and walk out thinking, "I'm sorry, did we read the same book?" Characters have disappeared or been created, entire story arcs were cut or doubled in size, moments of deep symbolic meaning or absolutely no meaning whatsoever materialized from nowhere while deeply meaningful moments vanished in a puff of smoke.
I think we've all been there. And, if you're like more, you tend to leave the theater in something of a huff. It's very rare that I leave a theater having seen a flick that made radical changes to the text where I didn't think, "You're not seriously claiming that was based on the book? You barely nodded to the original text!"
Because for me, there's a line between Based On, Inspired By, and Loosely Connected To (See How We Kept The Names the Same).
I'm not saying I've never seen it well done. The movie Stardust in 2007 got very free with Niel Gaiman's novel -- a book I liked quite a bit -- and when I saw many of the changes, I said, "Nice. I see why you did that." Because many of the adaptations were made as concessions to the medium rather than in total disregard for the original text.
On the other hand, Elton John's rock opera version of Aida is, to my mind, more Inspired By Verdi's original than actually based on it. They cut 3 of the acts, change most of Amneris's character arc, and rewrite a fair amount of Radames's backstory. But, you know, they kept the names the same. It's a nice reference, but, to me, almost a different play entirely.
Captain Film Major tends to come to the defense of adaptations. He says I expect them to bear too strong a resemblance to the book. Maybe I do. But that's because I have a very strict interpretation of the line between Based On and Inspired By. I think when you start getting jiggy with the plot, you're crossing over. He doesn't share that opinion.
For the first time, the project I'm working on was inspired by certain source material, and I chose to work closely with that material in the creation of the text. On the other hand, as the piece grew on its own, it moved away from the original source. I'd already changed most names, many motivations, a fair amount of characterization, and the ending. I think I've moved from Based On to Inspired By and, who knows, maybe even closer to Connected To (See How I Make All Those References). But that's just where the project wants to go.
How about you? Where do you draw the lines of Based On and Inspired By? Do you ever do Based On work? Inspired By?