Re the Devil's Interval, it's not a note but a gap of three tones between notes. A diminished 5th (or, if you like, an augmented 4th). Transpose a part by three tones and play against what was written originally and you get permanent dissonance.

Listen to the band The Devil's Interval doing Long Lankin and be very, very scared.

Personally I can't imagine being able to play music without sightreading it first. Even when I can see/hear in my head perfectly well what the line should be, I need to write it down first.