Halldorsson dived to his right to save Messi's second-half penalty, which prevented the South Americans from going 2-1 up in Moscow on Saturday.
"I did some homework because I knew this was a situation that could come up. It was a long shot but it happened," the goalkeeper said. "I looked at a lot of penalties from Messi and I also looked at how I've been behaving during the last couple of penalties.
"I tried to get in their minds, so they'd be thinking about me. I had a good feeling he'd go this way today. We were playing against one of the best teams in the world, who had the best player in the world.
"It's especially important for us because it helped us get a point to reach our goal - to get out of the group."
Iceland's next match is against Nigeria on Friday.