I want to begin the piece carefully, by considering my options and plotting a sensible route. But I can’t design from the top down, only from the ground up, so I begin playing to hear where that takes me. Whenever you’re in doubt about what to do or how to proceed, just start playing.
By playing I mute my sense of knowing ahead of time where I’ll go with the piece.
By playing I listen to what I’m able to do now, instead of what I think I could do someday.
By playing I can change course at any moment
if that sounds like a good direction to go in.
By playing I can enact and refine my capacity for responsiveness,
for tuning in to what’s happening.
Playing feels risky because my playing won’t be perfect, and my thinking won’t be airtight the way a printed score presents it. I’ll make mistakes, fudge notes, and head down dead ends. I’ll wish I had lingered longer on a tone, or passed by it more swiftly to get somewhere else to say what I really meant to say. Had I written a part that tells me where to go, I could have avoided these mistakes. But there is a flip side to playing without knowing where you’re going, which is conveying yourself to the listener.
I’m not hiding behind theory, I’m audible in my practice. We’re in this together because you can hear my process bringing me imperfectly from one moment to the next. It’s not everyone’s music, but for me improvising always feels musical.
brettworks 
I suppose there are dangers too in composing through improvisation, and for a long time I was embarrassed because I rarely composed in my head or on the page. Somehow I thought this isn’t the RIGHT way. Still, as you point out beautifully, it brings powerful new things to the task.
Well said Frank. I used to think that when I improvised I hadn’t “thought through” the music enough, but now I realize that it’s the surest route to surprising myself, which is a useful starting- and sometimes end-point for an eventual piece.