The Sussman's keynote at re:Clojure on the concept of layering from SDfF:
https://www.youtube.com/watch?v=8q8110rRTCA&t=36169s
Seems to be the same as his ICFP Scheme talk, which (along with John Cowan's talk on R7RS-large) was not posted online for some reason even though all the others were.
Anyway, it was also accompanied by an interview with the man himself:
https://pod.link/1471141263/episode/e29bac1bc31b7bd61e693d7daca1e3f6