Casillas impressed in their Champions League draw at Juventus on Tuesday.
"It is not easy to play games like this when there's so much time that you are not playing," he said.
"You notice that you are lacking certain things when there are two weeks between matches."
Casillas continued: "You have to be at your best when you get the opportunity, so that the coach can see that he can trust you.
"It is not easy because you play every two weeks and it is difficult to keep your focus, but with everyone helping me out it is easier to deal with it well."
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