It’s possible to run past the fella guarding the swamp without talking to him. He should call out to you if you don’t.
As you walk past him: “Hey there.”
As you get halfway between him and the bridge: “Whoa, hold on a second!”
As you hit the switch to summon the platform: “Just a quick word before you head that way!”
As you jump on the platform: “There’s a whole lot of trouble out there!”
As you jump off the platform to the other side: “Eh. Suit yourself.”
If you’ve triggered the second quote and talk to him, he skips his usual introduction and instead says:
Whatran: I would not recommend heading that way.
Frogatto: Why not?
Whatran: We’ve got a bit of a mushroom problem.
Frogatto: Mushroom… problem?
Whatran: Oh. You haven’t heard?
Frogatto: How much trouble can a mushroom make?
Whatran: Well, I’ll get to the point, since you seem pressed for time.
Whatran: Basically (short version, player can’t choose to hear the long explanation)
And he should have extra dialogue if you talk to him after killing the mushroom boss. This should work, regardless of whether the player talked to him or not:
Whatran: Wow, you’re back? What happened out there?
Frogatto: I handled business.
Whatran: What did you see?
Frogatto: Awfulness.
Whatran: Can you elaborate?
Frogatto: I don’t want to. It’s been a rough day.
Frogatto: And to be honest, I don’t see why you’d want to know.
Whatran: I’m sorry. I just want details in case we have to fight these things.
Frogatto: Don’t worry. The worst thing I saw out there is dead.
Whatran: Okay, I’ll leave you alone.
Whatran: But whatever you did out there… thank you.