How does his conversation with Cassius most affect Brutus?

Unclear question. However, I inferred you are referring to story "Julius Caeser" by Shakespeare.

Answer:

He rejected the proposal initially.

Explanation:

Remember that Cassius was among the conspirators who wanted Caeser dead.

So he tried persuade Brutus to join them in their plot to kill Ceaser. However, after the conversation Brutus wasn't affected to join them at that moment.