Robert Downey Jr. on Being 'Iron' Daddy

On a recent visit to the Iron Man 3 set, ET's Nancy O'Dell got Robert Downey Jr. to open up on the birth of his most recent child and what it's like to reprise his superhero role.

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"I think the best part about being a dad is when your wife isn't mad at you, which means you're doing your part," RDJ, 47, joked.

The Golden Globe winner became a dad for the second time in February when his wife Susan gave birth to a boy named Exton. While RDJ has been re-adjusting to life with a baby, he's also had to get reacquainted with the Iron Man costume, in his third hurrah as the comic book character.

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"[It's] more comfortable every time," RDJ said when asked how it feels to be back in the Iron Man suit. "They keep designing it to make it more sustainable so we can do more stuff in it."

Gwyneth Paltrow, who plays Pepper Potts in the film franchise, acts opposite RDJ as his love interest, but as they're both happily married, their off-screen relationship is much more innocent.

"He's like my big brother at this point," said the Oscar winner.

Marvel's Iron Man 3 opens May 3, 2013.