Caprice number 39 is such a sweet, gentle piece - it was one that took no effort to practice, even though it's in F# major. Perhaps that was the point: dress up an unagreeable key in a beautiful melody!
Caprice number 40 was difficult to learn but ultimately fun and rewarding. As musically simple as it is, I think it's one of the flashiest pieces in the book. (Case in point: my recording engineer, Stuart Breczkinsi, decided this one should be the background piece for the introductory video to my project.) The key of B Major makes the notes bright and cheerful on the viola. And the string crossing sections almost give it a fiddling/bluegrass feel (mm.27-37) . Of course, that style emerged from Baroque violin technique. Don't be fooled by the eighth notes: this one should fly off your fingers (and bow) as the Vivace assai marking requests. As with any fast piece, practice working up your speed in very short bursts, at times only a measure. Always remember to land on the next downbeat so you can thread your section practice together into longer fragments.