most important thing that you need to know about your opponents at public freerolls is that the most of them can be put in two general categories,
1. those who dont have a clue about poker
2. those who dont give a crap about the tourney they play
You probably guessing that if you wanna do good at large field FR you shouldnt belong into any of those two categories...
there is a more ways to play them, one is to play really really tight range of hands and to play them really aggressive, don't hesitate to over bet every time you bet, most people wont find it strange, even if they do no problem because you'll only play strong cards, don't even try to
bluff unless you know that the player you try to bluff knows where is the fold button... as the blinds go up and when the bubble is approaching you should start stealing the blinds but dont get to crazy with it do it from position and when you realize that players are tightening up... during the bubble time you should play more aggressive because that will give you the chips needed to play after the bubble. At that time you need to remember one general rule, better once in a top 3 than 10 times at min ITM...
GL at the table, hope i helped