Before I could expostulate he had spoken again.

Postulate suggest or assume the existence, fact, or truth of (something) as a basis for reasoning, discussion, or belief.
Protest : a statement or action expressing disapproval of or objection to something.
Explain : make (an idea or situation) clear to someone by describing it in more detail or revealing relevant facts.
Disclose : make (secret or new information) known.
Expostulate : express strong disapproval or disagreement.

Synonym of Expostulate is Protest