... I'm back to thinking about improvisation. Here's an interesting discussion from somewhere different, the world of recorders. This is Sarah Jeffery, a recorder player who runs a well-loved YouTube blog, working with Erik Bosgraaf, one of the most respected recorder professionals floating around at the moment.
Erik and Sarah makes some comments about improvisation: knowing where you start, and where you're going; not playing a note unless you know in your head what it's going to sound like. Their improvisation is also particularly successful because they've decided what they intend to do (the style in which they're playing, the available notes) before they start. Limitations, not freedom, generate successful improvisation (that's my take on it, anyway; their variations on La Folia are strengthened by the fact they're obliged to work with the scaffolding of the La Folia bass).
A recorder is a simple thing compared to an organ, but it's sometimes easier to recognise how a thing works when it's distilled into simple form.