The aforementioned shower insight: the slipbox is an "unbound" commonplace book with abstract ordering, but the material that enters the slipbox is still subject to the same "resonance" or feeling of inspiration of any commonplace book. Inspiration is the alpha and omega of creative work. You must be inspired to remember, voluntarily or involuntarily, a quote or a scene, and then record that on paper or on the computer. (As a rule, my plaintext commonplace file only includes quotes and scenes I have remembered, while away from the work -- not ones I think are significant at the time of reading or even studying.) Then, you must be inspired by events, topics, new learnings, and of course excerpts, to write. Everything in between is mechanical.
"[Growing up] is hard and nobody understands." // https://www.homestuck.com/story/2391