Giggles

[Eomer of the Rohirrim]

It seems to be used in lightening the mood - in places where the mood shouldn't be lightened. Fellowship cannot be let off here. When Merry and Pippin are captured, Aragorn should be devastated. Instead he makes a smart action-film quip, a winning smirk, and off he goes.

When Denethor loses his wits it should be a moment of high drama as Gandalf realises that it's up to him now. Instead, he assaults the Steward in a comic fashion and shrugs his shoulders as if to say "Bloody hell, more work!"

The comedy is implicit throughout most of the book; it didn't need to be forced. Treebeard's manner of speaking just is funny; you don't need to write jokes for stuff like that.