Exclusive: Robert Downey Jr. reveals the toughest part of playing Iron Man

With two solo movies and a worldwide smash team-up flick under his belt as the armored Avenger, you’d think Robert Downey Jr. would have the whole being a superhero thing down pat. But even while shooting this summer’s “Iron Man 3,” there was still one part of the process that tested his endurance and his patience.

That would be what Downey likes to call his “irritation therapy.”

[Coming Tuesday: Exclusive 'Iron Man 3' trailer premiere - Only on Yahoo! Movies]

See, when Tony Stark is inside his armored suit, you don’t see his face behind his mask. So filmmakers invented the HUD (Heads-Up Display) view. That’s when the camera goes inside the suit in an extreme close-up of Downey’s face with all of Tony's high-tech graphics floating around him.

But, Downey told Yahoo! Movies, creating the illusion is one of the toughest parts of the job.

“When you do the HUD work, usually it's kind of the last thing in the schedule,” Downey said. “And you're going back and essentially living the movie again in close-up, tired.”

You read that right: perhaps for the first time ever, an A-list, world-renown movie star feels he has too many close-ups.

Asked if the claustrophobic process--which involves him alone in front of a blank background with a camera in his face--got easier with each film, Downey said, “No, it didn’t,” and explained why it was so tough for him.

As an actor, Downey thrives on interacting with his costars and being physical, and filming the HUD shots requires him being isolated and immobile.

For example, when Tony believes his ladylove Pepper Potts (Gwyneth Paltrow) is about to die. The director would bark, “‘Oh, the most important woman in your life is falling off crane into a fiery pit. Okay? So, let’s just rehearse once and then we'll do it about 10 or 12 times until the camera is right and you’ve given enough.’ They’re just screaming direction at you... It’s like irritation therapy.

“I like the scenes. I like the action.”

Still, Downey did admit that going back to shoot the HUD segments at the end of production gave the team the ability to refine and improve scenes. “We had the luxury of being efficient enough to sometimes say, ‘Stop, stop. Wait a second. Let's not just shoot these next 20 lines. Let's look at the scene and how can we make it a little better or different.’”

So how many days has he spent filming the HUD shots during his four films as Iron Man? “Not enough for my tastes,” Downey snarkily replied.

He did say that it has gotten easier over time to actually wear the physical Iron Man suit. He said the version he wears in the new movie is “lighter; it's more flexible.”

But he made sure to clarify that “easier” was not the same thing as “easy.” He joked, “There is no comfortable version of it, so it's kind of like, ‘Hey, don’t you think these bamboo shoots are actually a little less rough on the cuticle until they get down to the nerve?’”

“Iron Man 3” opens everyone on May 3.

[Related: Maximum Marvel - See the latest superhero trailers, photos, and news]