How we cite our quotes: All quotations are from the Japanese to English translated subtitles of Seven Samurai.
Quote #1
MANZO: But what else can we do? We can't find any samurai. We'll bargain all right, whether we like it or not.
The peasants' duty is a little different than the samurai's duty. The samurai are doing it for some higher ideal, but the farmers are doing it because it's literally a matter of life and death. "Duty" becomes "an obligation to survive" for them.
Quote #2
KAMBEI: This is the nature of war. By protecting others, you save yourselves.
Kambei cuts to the heart of duty here: in the end, you do best for yourself when you stand by someone else. If you won't do it because it's the right thing, do it because it will help you stay alive.
Quote #3
KAMBEI: The reward will only be three meals a day and the fun of it, if you want to put it that way.
"The fun of it" in this case may be a coy way of saying "because we're obligated." The question becomes whether they're obligated because the peasants need their help, or because it will help them feel like they once did when they were proper samurai.