The word "大谈" is formed by directly combining the character "大" (dà), meaning "big" or "great", with the character "谈" (tán), meaning "talk" or "discuss". In this construction, "大" functions as an adverbial modifier that intensifies the verb "谈", yielding the combined meaning of "to talk extensively" or "to discuss at length". This formation relies on the literal semantic contributions of each character to create a specific verbal phrase.