Final answer:
The vocal technique used in Arnold Schoenberg's Pierrot lunaire is Sprechgesang, a style that combines elements of speaking and singing to evoke a specific expressive quality in avant-garde music.
Step-by-step explanation:
The innovative vocal technique featured in Arnold Schoenberg's Pierrot lunaire is called Sprechgesang, which means "speech song" in German. This technique involves a cross between speaking and singing where the vocalist hits certain pitched notes but does not sustain them in the traditional sung manner. Instead, the notes are delivered with the rhythm and cadence of speech, which allows for a great expression of the text being performed. In Pierrot lunaire, Schoenberg used this method to create a haunting and expressive mood that aligned with the avant-garde nature of the music. This work, along with his exploration of atonality and the twelve-tone scale, showcases his departure from traditional western music scales, involving dissonance and unanticipated notes to challenge the listener's expectations.