[Elladan and Elrohir]

Well, in the book Gandalf sees Treebeard walking in Fangorn and later apparently spies on him, Merry and Pippin, because he hasn't met them but somehow knows that they have met. He doesn't actually meet up with them until he comes to Isengard after it has fallen to the Ents. Gandalf seems to know more than one might expect, and less than one might think.

I thought the whole "let's go see the white wizard" thing in the first scene with Treebeard was a pretty dumb way of trying to build up suspense, if for no other reason than that Treebeard would never refer to Gandalf as "the white wizard."