vacillating between silly and serious, the talk with Ansari is interrupted several times with giggling—most of it his own.
vacillating and unintelligent, his actions were all guided by his mistress, Mme Bonnemain.
1590s, "to sway unsteadily," from Latin vacillatum, from vacillare (see vacillation). Meaning "to waver between two opinions or courses" is recorded from 1620s. Related: Vacillated; vacillates; vacillating.