Ron honestly expected a spider, but instead he gets a wall of rubble, and people are laughing, but not Harry. Harry remembers that wall of rubble too, remembers Ron's strained voice from the other side of that wall in a dark and wet tunnel as Harry goes to face a Basilisk alone. When he meets Ron's eyes, he just nods slowly because now harry knows that Ron's greatest fear, for all that his friend hates spiders, and struggles for attention, is being helpless too aid those he cares for. So Ron looks back at Harry with steely eyes, unashamed of his fear and the two boys share a moment of understanding, much to Hermiony's unhidden befuddlement.