Finnick freezes visibly when Katniss announces that it's the Quarter Quell.

"How do you know?" he asks immediately, the question slipping out of him before he's able to consider it. Was she just assuming that it was the Quarter Quell because it was an arena that was unfamiliar to her? Had Plutarch shown her the plans before she went in; that doesn't make any sense, because as far he knew, she wasn't clued into the plan. Haymitch had been so sure that she'd wear everything on her sleeve and end up exposing all of them. Hell, Plutarch hadn't even shown him and Johanna the plans, and they were some of the first tributes on his merry bandwagon of rebellion.