Wow; one of my favorite nocturnes!I'm not 100% familiar with the score, but I assume you're talking about the 5 minute mark in this recording:As Chopin himself said, "Suppleness above all," so make sure your wrist is reasonably flexible, so you can transition from one note of the melody to the next. It helps a lot as well to not lift the fingers to high when trilling- it's also unnecessary for the "musical effect" in this section. The arm shouldn't play much of a role either, but it should still be relaxed to help carry the hand to the next note/trill.I agree with the edition in that video as well; use 1 to highlight the melody, then switch to 2-3 for the trill (or 2-4 if that's more comfortable for you/if 2-3 gets frustrating).