This is the 6477th most frequent Japanese word.
セリフ
A line or dialogue spoken in a play, movie, or conversation.
Here, 'セリフ' refers to a spoken line in a play or movie, indicating its role as dialogue.
彼のセリフは感動的だった。
His dialogue was moving.
In this sentence, 'セリフ' is used to describe memorized lines that actors say in performance.
セリフを覚えるのは難しい。
Memorizing lines is difficult.
Here, 'セリフ' is used more generally to mean any set expression someone says, emphasizing its predictability.