Los Angeles, July 20 (IANS): Hollywood action heroes Sylvester Stallone and Arnold Schwarzenegger may be serious while shooting together, but they are quite the opposite off the sets.

The two stars were spotted together during screening of their movie "Escape Plan" at Comic-Con in San Diego Thursday.

"I'm just here to irritate him. I was put on this earth to irritate him," eonline.com quoted Stallone as saying. He also said that both have been known to be practical jokers.

Schwarzenegger says "that's part of it, to play tricks on each other".

"That's what makes it fun. But when we were on the set and when we were shooting, it was serious business," he added.

The "Terminator" star also recalled how they used to compete against each other on screen.

"We've competed against each other our whole career. I mean, we were trying to outdo each other about who uses the biggest guns, who has the most cut muscles, who has the least amount of body fat, who has the biggest muscles, who kills more people on the screen," he said.

But this competition helped them to make their forthcoming film, which is releasing Oct 18, better.

"It was really great, because we could compete within the movie and the movie really benefits from that because I was really trying to pull out my best performance because I wanted to outdo him. And he pulling out his best performance, so it really helped the movie," said Schwarzenegger.