The Real Madrid goalkeeper saved Joao Moutinho's spot-kick, and saw Bruno Alves' strike rattle the crossbar before Cesc Fabregas converted the winning penalty.
"I was very quiet, or so it seemed to me," Casillas told Telecinco.
"Penalties are a lottery. I had the intuition in that penalty for Portugal and I stopped it, which was the best we could get.
"It was a kind of revenge for what happened to us as team. Ronaldo? He always shoots well in penalties. It was a decision from Portugal that he didn't shoot.
"[Sergio] Ramos' penalty? Well, you really need guts to shoot on Panenka's style. Ramos has showed male attributes here, and the courage he got.
|Spain 0-0 Portugal
"We were tired in the game, and that counts. Now we have another day to rest, and let's see what happen in the final.
"We've made history, already. We are still making it. We hope people remember that forever. Because it is unique what we are achieving."
Fabregas, meanwhile, did not hide his enthusiasm to reach the Euro 2012 final following his winning strike in the shoot-out.
"The team has worked so hard," the midfielder stated. "I had the feeling that there would be penalties tonight.
"I wanted to take one if we had this opportunity. To be honest, I am very excited. To reach another final in four years is amazing."
Spain were on the back foot during some stages of the match, but Sergio Ramos thinks that his side worthy winners.
"After a very long season with so much work, we deserved this," the Real Madrid defender said.
"We are happy with the performance of the team. Individual success is not as important as the collective one, which is to reach the final.
"I am proud to be Spanish."
Left-back Jordi Alba thought that the match was finely balanced between the two sides, but praised the way Casillas rose to the occasion.
"To be honest, it was a very even match, both in 90 minutes and the extra-time," Alba said. "Spain was the better team in extra-time, and before it was a very tough game.
"Iker Casillas is always here, and he's a huge keeper. We knew he was going to appear, like he always does
"The ones who really know take the penalties. I could have, but there are more players who do well."