Do you prefer creating on the design wall or from a sketch?
They work hand in hand. I may sketch out something, or collect scraps from other mediums and put them in the sketchbook for safe-keeping until I've developed the idea or collected other supplies enough to take it to the design wall for the next step of the creative process. Then I can start to play ... auditioning fabrics, yarns, scraps, etc. Trying to figure out what will work. For quilts the design wall has been wonderful to layout the blocks and let them stew a while before I actually sew them together. I guess I could say the same for the smaller journal quilts. Even the journal quilts usually start as a sketch, though.
This post from 2011 about the Bluebird Journal Quilt is a good explanation of how I tend to work.
Is working on a Design Wall more like improvisational jazz, offering more freedom for the piece to grow and develop as it wants?
It can be--although it's hard for a perfectionist like me to let loose and just let it flow like jazz. At some point, you can get into "The Zone" and just let it come ... I tend to be more deliberate and take more time than being in the moment allows. Usually, I have something in mind before I start, something that's been percolating for some time on the ideas level. For the bluebird quilt above, I thought I wanted to use that lovely pink snow dye, but it didn't make it into the final piece. I used the dyed cheesecloth instead because it was more opaque -- like a tree full of spring blossoms is opaque, allowing you to see the sky through all those sweet flowers.
Does a sketchbook lock you into a design that may or may not be possible in real life?
I don't usually feel like a sketch locks me into the original design--Of course, I might feel differently if I were designing a building with millions of dollars were at stake. To me the sketchbook is just a step in the creative process. A placeholder for possibilities, rather than a blueprint etched in stone. I'm usually working out the process as I go along, and wouldn't necessarily know that I needed to do B before A until I get there.