Everyone should agree in the plan. Which means sharing what the plan is. Which really means the bossiest person tells everyone what the plan is.
Commentary by memnarch (who has not seen the movie)
It's probably the dialogue and me reading into it, but again, great expression by Chewie. I can just see the "are you kidding me" expression Ben is giving Jim, who's busy contradicting Annie's more sensible plan. Admittedly, it's the plan of the player playing the known spy, so Jim would have a good point IF Jim wasn't already known for not thinking things through when playing in the game.
However. When you're with people you've been around for a long time, you can start playing into and subverting their expectations. And one of the things with Jim is that he did manage to pull off being a double/triple agent for a long time as Han, so when he puts his mind to something, he'll get results. So.... could this be something to just throw Finn off the scent? Or something to make him obviously pick a side perhaps by assaulting the First Order? It's definitely easier to trust someone who cuts ties cleanly like that. And if Finn doesn't shoot to kill, well, that's why Zeppo has a gun of his own and Chewie for backup.
Finn: All right, here’s the plan. We approach—
Zeppo: I come up with the plans around here, kid!
Finn: Uh... okay... What’s the plan?
Zeppo: What do you think it is?
Finn: We approach the door and sneak in quietly?
Zeppo: Well, that’ll never work.
Finn: Why not?
Zeppo: They’ll hear us as soon as we start blasting!
Finn: Blast? Why would we go in guns blazing?
Zeppo: Because that’s the plan! Glad you agree!