In the book Catching Fire characters are committing acts of defiance.  

Katniss was defiant during her private session.She thought she wanted to "break through the smug veneer of those who use their brains to find amusing. To make them relive that while they were vulnerable to the Capitols cruelties, they are as well." Its defiant to show that she can do more than they can. 

Peeta was defiant during his private session because he painted rue after she was covered in flowers. This is defiant because he's reminding them that they killed innocent children.  

The tributes were defiant because  they remind the capitol citizens that they are going to be killed. This is defiant because they try and stop the games and turn the capitol against Snow. "The Capitol must be suffering because they will lose us." 

Cinna was defiant because he turned Katniss' wedding dress into a mocking jay dress. This was defiant because the mocking jay is a symbol of rebellion. "Because Cinna has turned me into a mocking jay." 

In conclusion I feel the biggest act of defiance was Cinna turning Katniss' dress into a mocking jay. I feel like this because he was showing he supported rebellions and people against the capitol.