I'm currently on my second adaptation of a novel, and the whole process is fascinating for me.
I saw Wicked recently, and had read the book beforehand. I've also read all the Harry Potters, and subsequently saw the movies. The act of adapting from one genre to another seems to swing from something quite different to the source (Wicked) to something very faithful to the source (Harry Potter). Right now, this act of adapting has settled with me thus -
I treat the novel as source material, in the same way I treated historic documents about Horatio Nelson for my musical about him. In the same way I treated research about clockwork automata for a show about a clockwork woman. (Ref 'Men vs Women': when I do write women, I make them not be human!)
In other words, I take what I want from it and ignore the rest. I have a very short attention span, so if something from the source instantly grabs me, I keep it. Anything I can't instantly recall, I disregard. Well, at least in terms of the global structure of a show. I sometimes go back if I want a specific tiny detail, but for the most part, once I'm away from the source, I'm away from it. Beyond it.
For me, the intention is not to faithfully recreate someone else's work in a different genre. I want to create something entirely new. I'll cherry-pick at the source, but only to serve my intention for this new work I'm creating. If it doesn't serve my story, out it goes.
Frankly, I don't see how an adaptation can, or should even attempt to be totally faithful to the source. There are writers involved in an adaptation, and as soon as you get creative people involved, they can't help but filter the story through themselves. It should be new, and true to its creators.
After all, even Shakespeare had source material.
