The Czech became a second half substitute as back-up keeper Ross Turnbull was red carded but Cech saved the resulting penalty from Leighton Baines as the Londoners eventually won 2-1 in extra time.
Cech said: "Sometimes that is the way it is. You are sitting on the bench not even moving and then suddenly you have to get up.
"When you go in and you are facing a penalty kick it is not an ideal situation, but I was ready.
"I was watching the game and watching what was happening. I had an idea when I went on what was going to happen.
"As soon as the referee pointed to the spot I knew I had to go on. I was getting ready to think what Mr Baines could do."
The 29-year-old added: "The first penalty save helps and the rebound helps even more.
"Then within 90 seconds there was another save, so then I said I was warm enough!
"I was really pleased because it is the first time this has ever happened to me, to go in like that. It was really good."