Only ET is behind the scenes of 'Iron Man 2' with its invincible stars! Robert Downey Jr. and Gwyneth Paltrow return with new faces of the franchise, including Mickey Rourke, Scarlett Johansson and Don Cheadle -- and we have exclusive details of the film's fiery action and steamy love triangle!

"We have been blowing things up, and I have been trying to avoid [Whiplash's] laser whips," Robert tells ET. "Come on, it doesn't get any better than this."

In theaters May 7, 2010, 'Iron Man 2' follows the further adventures of billionaire industrialist and weapons manufacturer Tony Stark (Downey Jr.). Now that the world knows his secret identity, he finds himself facing-off against Russian baddie Whiplash (Rourke), devious multi-billionaire businessman Justin Hammer (Sam Rockwell) and the seductive Black Widow (Johansson), who is giving Pepper Potts (Paltrow) a run for her money. But will Tony Stark go it alone? The film is once again helmed by Jon Favreau.

"Robert's the soul of the franchise," says Jon, who also returns onscreen as Tony's driver, Happy Hogan. "Every day we come to the set we try to discover new ways to make the story interesting and unexpected."

When we visited the 'Iron Man 2' set, they were recreating the Monaco Grand Prix, and Robert fills us in on the details: "Tony Stark has come to Monaco, supposedly to observe the race, but then, as it happens, he is racing in it. This is where he meets his nemesis, and this big kind of action sequence unfolds on the track," he explains. "This is probably about my favorite thing in the world to do professionally because you get the free adrenaline rush and it's all safe."

Robert reportedly worked out for five months to gain a beefier 15 pounds of muscle after slimming down for his role as 'Sherlock Holmes.' "You do it smartly and safely," says Robert about his fitness regimen. "I love fitness, I love martial arts, I love choreography."

Gwyneth assures us that, "Pepper's evolving a little bit in her look -- she is still rocking all the big high heels," and shares her health regimen: "What I have discovered recently, which has been a really important thing, is doing cleansing. I can't believe that I can put on a bathing suit or wear these costumes that are tiny and short and feel good about myself. But it takes a lot of effort, you know?"

And Scarlett is definitely turning heads with her Black Widow look: "I definitely adopted a really healthy lifestyle, did a lot of strength training and just core work and all of that stuff," she says. "My character comes on the scene and she is sort of the new kid at school with the cool clothes and the fancy backpack and all that stuff. There is a little bit of the 'who is that?' coming into this circle."

A love triangle develops between Pepper Potts and Black Widow with Tony Stark, and Robert laughs, "Like I would ever have Gwyneth Paltrow and Scarlett Johansson fighting over me -- all I know is that Susan Downey gets to watch it all at the monitor. ... She says it's okay."

"He loves it; it's his favorite thing," laughs Gwyneth. "When Scarlett and I are both on set it's like his favorite day."

Watch ET for more 'Iron Man 2' action!