In the book "Catching fire" During Katniss's private session, she hung a dummy of Seneca Crane.Katniss wanted to "Break through the smug veneer of these who use their brains to find amazing ways to kill to make them relive that while the tributes were vulnerable to the capitols cruelties the game makers are as well" This is defiant because she's reminding them that she beat the system/game makers.  

     During Peeta's interview he told everyone that Katniss is pregnant. This is an act of defiance because a couple of seconds after he told everyone they started rioting and they are super mad and want the games to be cancelled.  

     The tributes held hands in unity showing they are friends and not enemies and they are suppose to want to kill each other in the arena but they are friends. This is an act of defiance because they are suppose to want to kill each other but they want to join forces. 

     In Cinna's act of defiance he made Katniss's dress into a mocking jay. This is a act of defiance because the mocking jay is a symbol of rebellion.