Harry Potter and the Sorcerer's Stone
How we cite our quotes:
You might belong in Gryffindor,
Where dwell the brave at heart,
Their daring, nerve, and chivalry
Set Gryffindors apart… (7.33)
Just as the Hufflepuffs are defined by their loyalty, according to the Sorting Hat, the Gryffindors are defined by their courage. Specifically, their courage, which has aspects of "daring, nerve, and chivalry," is founded in the heart – not the mind – it's emotional first, calculated second. In other words, Gryffindor bravery is sometimes brought on by feeling, rather than thinking.
"Malfoy tricked you," Hermione said to Harry. "You realize that, don't you? He was never going to meet you – Filch knew someone was going to be in the trophy room, Malfoy must have tipped him off." (9.169)
Malfoy, as a Slytherin, is supposed to be more adept at cunning than courage, and this moment only proves it. He didn't just chicken out of his midnight duel with Harry; he never planned to go at all. It wasn't meant to be a test of his courage, but a test of Harry's gullibility.
Harry then did something that was both very brave and very stupid: He took a great running leap and managed to fasten his arms around the troll's neck from behind. (10.123)
Here's where that idea of being "brave at heart" (7.33) kicks in again. Harry attacks the troll by being "both very brave and very stupid" – but that stupidity is almost necessary in order for him to be able to act bravely. If he'd stopped to think about it, he would have probably realized that jumping on a troll is not high on the list of acts of self-preservation.