Fagen and Becker were ruthless perfectionists, editing down guitar solos of the best players down to a few notes, swapping entire bands overnight, or adding a few high (1970s) synthesizer notes to make a flute sound a bit fuller.
Only when you get to a point that is beyond perfection can you start to improvise to give things that personal edge. And that is exactly the same for presentations: only when you have rehearsed in and out, you can deliver that truly relaxed and spontaneous presentation.
The movie is on Netflix, but to my surprise I also found a lower-quality version free online, the site seems legitimate:
Watch Classic Albums - Steely Dan - Aja in Music | View More Free Videos Online at Veoh.com