What makes Marvel's superheroes so cool? Besides their drive to save the universe, their fancy moves and action scenes, it's their costumes that complete their fierceness factor.

In the "Avengers: Age of Ultron - Official 'Costumes' Featurette," Writer/Director Josh Whedon and Costume Designer Alexandra Byrne discuss what's involved with the making of the characters' costumes.

According to Yahoo.com, "Scarlett Johansson's Black Widow suit got the most dramatic alteration. 'I went electric,' said Johansson, who reprises the role of Black Widow (her fourth time as the Russian heroine) in reference to the Tron-esque new suit (courtesy of Robert Downey Jr.'s Tony Stark) that allows her to fire electrical blasts of energy at the bad guys."

Comfort wasn't a luxury that she could have, however.

"'I had a giant battery pack that I dragged around everywhere,' added the actress, who was pregnant during the movie's filming (she delivered a baby girl last September)."

As for Chris Hemsworth, People's Sexiest Man Alive (November), who plays the muscular Thor for a fourth time in "Avengers: Age of Ultron, he got to show off his guns more this time around. Fans won't be complaining, that's for sure!

[I] lost the sleeves," Hemsworth told Yahoo.com. "It was too hot. There's maybe one scene where we pay homage to [the classic look]. But [the sleeves] were ridiculously hot and uncomfortable."

There is also "a lot of metal work involved" in his costume, reiterated Byrne. 

Hemsworth also points out that over all "it gets a little more comfortable each time" in the featurette.

"The Avengers: Age of Ultron" cast also includes: Robert Downey Jr. as Iron Man, Mark Ruffalo as Hulk, Chris Evans as Captain America, and Jeremy Renner as Hawkeye, with the additional support of Don Cheadle as James Rhodes/War Machine, Cobie Smulders as Agent Maria Hill, Stellan Skarsgård as Erik Selvig and Samuel L. Jackson as Nick Fury.

"The team must reassemble to defeat James Spader as Ultron, a terrifying technological villain hell-bent on human extinction. Along the way, they confront two mysterious and powerful newcomers, Pietro Maximoff, played by Aaron Taylor-Johnson, and Wanda Maximoff, played by Elizabeth Olsen and meet an old friend in a new form when Paul Bettany becomes Vision."

Check out the "Avengers: Age of Ultron - Official 'Costumes' Featurette." The film hits theaters on May 1.