In Canon Edit
Treebeard encounters Meriadoc Brandybuck and Peregrin Took after they escape from the Orcs' skirmish with the Rohirrim, and minds them on Gandalf's orders. He hears their news about the goings-on in the world beyond Fangorn Forest, which confirms his worst suspicions about Saruman's treachery. He then leads the assault on Isengard.
In the books, Treebeard is stated to be the oldest of all living things, older even than the Elves. He is ancient and wise, slow to anger, but an unstoppable force of nature once roused. He is not 'hasty', which is his favorite word for Merry and Pippin; rather, having been alive so very long, he knows that things must take their course, and going about in a great hurry won't change that. He makes important decisions with the full time and care that they deserve.
In the movies, his 'unhasty' nature comes off more like the dithering of a very old man rather than ancient wisdom. The fact that he seems to be clueless about what Saruman is doing to his forest until the Hobbits trick him into going to see the damage for himself does little for his character.