The hobbits miss the moment when Aragorn freezes. Rumours have reached him of Olog-hai, trolls that can briefly endure the sun. Has Sauron grown so strong that they have crossed the mountains? With Riders before and behind and Frodo wasting away before his eyes, it is easy to believe.
Before Aragorn’s inward eye, Bilbo’s face appears, recounting his story of the trolls. In the latest version, they had homely names like Bert and Tom. Aragorn had meant to go look for them, never found the time.
Now unconcerned, he picks up a stick and breaks it on the stooping troll.