User blog comment:Thailog/Ep. 126 "Auld Acquaintance" discussion/@comment-4584676-20120421185537/@comment-1038387-20120421190146

About Billy: he changes back and forth between himself and Captain Marvel by saying "Shazam!". See "Alpha Male" and "Misplaced". Zatanna's spell, for as far as we've been able to decipher it, was "Blurt out Shazam!". He did, and changed.