How many letters does the top answer for “a person who takes part in a conversation” have?

The most common answer is interlocutor (12 letters).

Crossword clue: a person who takes part in a conversation

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

interlocutor 12 letters wn-def

a person who takes part in a conversation

a person who takes part in social activities 50% similar → socialiser, socializer