Ronaldo has been the subject of barracking from Real fans this season.
Casillas told realmadrid.com: "Cristiano Ronaldo is a very competitive player who always tries hard and wants to do his best.
"People will always ask a lot of him because of the sort of player he is. He wants to do his best for himself and for the club. The Bernabeu crowd is very demanding, and that's a good thing. There is no debate about his quality. He has always had the support of the fans."