I just had a quick thought on this line, after listening to session 25. Professor Olsen was asking why Sam would show this skepticism that Old Man Willow threw Frodo in the water. One hypothesis is that he's comforting Frodo. I can see this, but I think instead that Sam doesn't really believe yet that the tree tried to kill Frodo. Sam realizes that the tree is singing about sleep, and he doesn't trust this. But I don't know if this adds up to him believing that the tree has murderous intent. Otherwise he probably wouldn't have gone off after the ponies at all, and would have stayed with his companions to protect them. So, I think he's aware that something uncanny is happening, but not the level of danger that they actually face. He seems to be disinclined to believe that a tree would be that malevolent. This may actually be connected with why Sam was able to perceive the singing in the first place. He seems to have more "sympathy" with Old Man Willow than the other hobbits, which could be due to his general kinship with plants? Maybe also his kinship with the magical/fantastical (Elves, sir!).