I find it fascinating how F-D's intonation changes with and emphasises the mood and individual words of the text.
I wonder how significant fluency in a language and, for a wider perspective, knowledge about language use and historical context of the composition is for the interpretation of a piece. I guess these kinds of things are what distinguishes the good from the best interpretations.
I always cringe a little when I hear Schubert lieder performed with foreign accent. I don't hear much difference between native and foreign speakers in Opera, but for Schubert Lieder it's somehow different.