A centerpiece of Marvel's "Avengers: Age of Ultron" preview at San Diego Comic-Con was something fans already knew a bit about. Early concept footage showed the Hulk fighting Iron Man in his super-sized Hulkbuster armor. But what Hall H saw from the finished fight went far beyond expectations.

When MTV spoke with Robert Downey Jr. after the panel, he said the fight scene was the result of years of planning.

Related: First-Look ‘Avengers: Age Of Ultron’ Footage Brings Down The House At Comic-Con

"I feel like I've been working on that sequence for two years, because it's been something we wanted to do," he said. "Then we thought, 'What should the sequence be?' Then we thought, 'You don't want to talk too much. There needs to be an element in it.' Of course, there's me and Mark just sitting around going, 'Wow, I can't believe this is actually happening now.'"

For as exciting as the concept of the fight might be, it comes at the cost of a friendship that much of the "Avengers" fanbase has come to love -- and that element is key according to Downey.

"It can't just be a good fight. It's got to be about something," he said. "The great thing in 'Age of Ultron' is that fight between them, nobody wins when two people who are friends fight."

"Avengers: Age of Ultron" opens in theaters on May 1.

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