Katniss knew what she was volunteering for--she'd watched the games, seen kids her own age go compete in them and die. She's seen how brutally horrible the games are, how some tributes die a slow death, tortured by the careers, and how some die quickly before they even realize what's happening. She sees Haymitch, the town drunk and the only still surviving District 12 victor who has no loved ones and who spends his days drinking, trying to forget the nightmares he experienced in the games. Katniss sees and knows all of this, yet when Prim's name is drawn, she doesn't even question yelling out, "I volunteer! I volunteer as tribute." Every single time I see that part in the trailer, I get chills. Every single time.
How far would you go for someone you love? Who could you love so much that you would be willing to volunteer your life into the Hunger Games for? It'd be a huge price to pay, going into that arena, with the odds in your favor of dying. Who, if their name were drawn from the bowl, would cause you to not even think twice and yell, "I volunteer! I volunteer as tribute."