I remember seeing a YouTube video about a park full of people just minding their own business. A man plays some music and starts dancing in the middle of this park. Of course, nobody paid him no mind. That was until another person joined the dance. And then another. And another. Next thing you know a huge crowd gathers around the spot and everyone is dancing.
I think the same thing happened here. Kurosawa was just telling his coworkers what he was planning which led to one guy being so motivated that he starts hyping up the fight. Now everyone is excited.