Harry was barely listening to her. He was turning the Snitch over and over in his hands, half expecting it to break open, to reveal the Resurrection Stone, to prove to Hermione that he was right, that the Deathly Hallows were real.
She appealed to Ron.
"You don't believe in this, do you?"
Harry looked up. Ron hesitated.
"I dunno ... I mean ... bits of it sort of fit together," said Ron awkwardly. "But when you look at the whole thing ..." He took a deep breath. "I think we're supposed to get rid of Horcruxes, Harry. That's what Dumbledore told us to do. Maybe ... maybe we should forget about this Hallows business."
"Thank you, Ron," said Hermione. "I'll take first watch."
And she(Hermione) strode past Harry and sat down in the tent entrance, turning the action into a fierce full stop.
Harry Potter and the Deathly Hallows
I'm not exactly sure of the meaning of "turning the action into a fierce full stop". Maybe Hermione just ended up in a full stop? What's "the action" referring to? How should we understand it correctly?