My advice to anyone adapting a novel is that once they've read it and learnt to understand it, then they must throw it away and never look at it again!
I don't think screenwriting is therapeutic. It's actually really, really hard for me. It's not an enjoyable process.
When I write for 'n+1,' I begin by doing a lot of reading, to try to convince myself I'm not stupid. Then I scribble down a paragraph here, a paragraph there, when a notion strikes. Then I see if I can arrange those notions in a way that yields an argument.