Using interactional video data from the Cha'palaa language of Ecuador (Barbacoan family), this talk discusses how a particular linguistic form, a clause marked by the verbal suffix -ba, grammatically encodes the conversational action of 'counter-assertion'. 'Counter-assertion'  is defined as contradicting or problematizing some element of the previous conversational turn, and relates to how participants manage common ground. The talk describes the diachronic development of the counter-assertive through processes of insubordination in which dependent clauses are licensed to stand as independent clauses in certain contexts, and how -ba eventually gained finite morpheme status, its interactive meaning becoming a grammatical option for main clauses. Examples of counter-assertion in excerpts from a conversational video corpus help to shed light on the interplay between grammatical form and conversational action, with implications that go beyond any one specific form or language.