According to Osho, the smile before speaking acknowledges the absurd, torturous necessity of using words to point to the wordless; it is compassion and a reminder of the silence behind speech. During discourse the smile fades because speaking is a burden. When discourse ends, the smile returns—freedom from words—hinting that the real teaching is the nonverbal presence you will learn to hear.
He smiles to show that the real teaching is silent; talking is hard but needed, so the smile appears before and after, and fades during the effort of speaking.