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."
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 stated
"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."
Courtois also outlined his hopes for the immediate future, adding that he would like to prolong his stay at the Vicente Calderon.
"I am very happy that the fans sing my name. I said that I want to stay at Atletico for another year, but I have to wait.
"I am not 100 per cent sure, but I would love to continue and I think it will happen," he concluded.
Starting last month and continuing for 10 weeks, Goal.com is publishing exclusive extracts from Luca Caioli's new book, Ronaldo: The Obsession for Perfection. The third extract, published on Friday, looks at Ronaldo's trial and subsequent signing for Sporting Lisbon at the age of 12. Don't miss it!