Paul Rudd knew some people would scratch their heads when they heard he would be Ant-Man, Marvel's newest superhero.

After all, his stock-in-trade had been to play a lovable goofball in comedies such as Our Idiot Brother (2011), Knocked Up (2007), the Anchorman movies (2004 and 2013) and the television show Friends.

But he got a kick from the unlikeliness of it all. "I enjoyed the fact that when I was cast people went, 'Huh, really?' I think that Marvel likes to do that and I was thrilled to have the opportunity."

Speaking to Life and other press in Los Angeles, the 46-year-old also talked about the buzz he got from seeing how excited his son Jack, nine, became about the film.

When he was first cast as Scott Lang - an ex-convict who becomes a superhero when he dons a suit that shrinks him to the size of an ant - his son pretended to be unimpressed, saying: "Wow, I can't wait to see how stupid that'll be."

But the boy was secretly thrilled.

Rudd says: "It's the first thing I've ever done, ever, that he is legitimately jazzed about. He can see it, his friends know about it.

"When we were at Disneyland, they had kind of a sneak preview of an Ant-Man event. As soon as it ended, he just looked at me and was like, 'That's awesome!'

"As a parent, to see the look on my kid's face when he's watching this… I'll never forget it."

Not only was he playing the lead role in a big-budget action flick for the first time, but he was also drafted to help rewrite the script in the final stages before production.

Rudd - whose writing credits include the critically praised but short-lived TV comedy Party Down (2009-2010), about a group of out-of-work actors forced to work as catering staff in Hollywood - is credited by Ant-Man producer and Marvel Studios boss Kevin Feige with coming up with some of the best parts of the film, which takes a more comedic approach than many of the studio's superhero movies.

Mr Feige says: "He and co-writer Adam McKay, along with the director Peyton Reed, did an incredible job of transforming the movie in the final months leading to the start of production. Almost all of the great and memorable parts of the film came out of those rewrites."

Rudd managed to overcome the fact that he knew next to nothing about the character of Ant-Man through research and then just getting into "the mindset as much as possible".

Another touchstone for him was the theme of parent-child relationships in the movie: Ant-Man's main motivation for putting on the suit is so he can rebuild his life and see more of his little daughter Cassie, whom he has had restricted access to since coming out of jail.

That father-daughter relationship "was the thing that I hung the whole movie on", says Rudd, who also has a daughter Darby, four, with wife Julie Yeager, a former publicist.

"You can have a movie that has amazing effects and brilliant visuals, a lot of action, humour, whatever… but whenever you see something which you can connect to, that's emotionally resonant, it stays with you in a very different way."

This being a superhero movie, there was inevitably a strong physical component to the role.

He saw his "dad bod" being transformed - a fact the rest of the cast clearly enjoys teasing him about.

During the light-hearted press conference, David Dastmalchian - who plays one of the ex-convicts who helps Lang break in and steal the Ant-Man suit - accuses Rudd of "enjoying being ripped" in a scene where he strips off his shirt and briefly exposes his buff chest.

"I was very self-conscious during that day of shooting," Rudd says.

Dastmalchian adds: "I remember what you had for lunch that day we shot your abs - you had one almond..."

"...and I felt so bloated," says Rudd, playing along.

Jokes aside, that brief moment in the film was the product of a punishing year-long diet and exercise regimen, which Rudd knew he would need if he was to fit into Ant-Man's body-hugging suit.

But he will admit that he loved wearing the suit that allows his character to shrink.

"I think it's the coolest-looking suit of all of them and I loved wearing it. It was not that uncomfortable," says the star.

"He would wear it all the time," jokes Michael Pena, who plays another ex-convict in the film.

"Even on my days off," quips Rudd.

On a more serious note, he adds: "There's something that happens when you get in that thing - I would stand differently, I would feel different. I'd feel like Ant-Man in that thing."

Ant-Man opens 20 years after Clueless, Rudd's breakout film, was released in 1995, establishing him as something of a heart-throb.

Says Evangeline Lilly, who plays the daughter of the man who invented the Ant-Man suit and Lang's love interest: "Who didn't have a crush on Paul Rudd in Clueless? So dreamy," she says, grinning at him.

Rudd says: "I know, it's crazy. I fell in love with me. It's unbelievable, isn't it?"

But he turns serious when he reflects on the two decades that have passed since.

"I am so appreciative that I've been able to continue to work doing something that I love. And not only doing something that I love, but also working on movies that I've loved.

"I want to work on things that I would want to see, and for a large part of my career, the vast majority, that's been true. And I'm just very, very appreciative of that."