The keeper, on loan from Chelsea, has impressed during his time at the Vicente Calderon and admitted that he looked up to his counterpart from the Santiago Bernabeu during his early years.
"He was my idol as a child. I like him a lot, but for all the respect that I have, he is a rival and I will always want to beat him," Courtois said in an interview with AS.
"I admire his speed under the posts. He has very good reflexes. He is a very good goalkeeper, in general."
||Atletico Madrid are 18/1 to beat Real Madrid 1-0 on Wednesday with Bet365.
Courtois was sent off after just 21 minutes when the two sides met earlier this season and was impressed that Casillas had taken the time to offer him words of comfort.
"Despite playing for Madrid and being one of the best in the world, he doesn't lose his good manners and that's something I like a lot about him. He is great on the field and also off it."
Atletico Madrid host Jose Mourinho's side on Wednesday evening and Courtois believes both teams will be giving everything to earn the win.
"I know the derby is a very special game for all the fans and for this club," he said.
"In less than two weeks they [Real] have to play the Clasico and they will not want to depend on the outcome against Barcelona.
"This derby will be a great match because we need the points as much as they do."
The clash at the Vicente Calderon kicks off at 21.00.