'Avengers: Age of Ultron': Here's why it will be darker than the first film

'Avengers: Age of Ultron' finds the titular superhero team facing off against a villain who was created out of artificial intelligence. 'It's darker, more emotional, and more based on the characters as people, as opposed to superheroes,' 'Ultron' star Elizabeth Olsen said of the film.

The caped crusaders are back in force in "Avengers: Age of Ultron," but this time, the evil they battle is bringing them face to face with the darker consequences of being a superhero.

"Age of Ultron," the sequel to Walt Disney Co's 2012 Marvel blockbuster "Avengers" and out in U.S. theaters on May 1, will see its star-studded superhero ensemble – Iron Man, Thor, Black Widow, Captain America, The Hulk and Hawkeye – tackle their own fears.

Writer-director Joss Whedon said he wanted to "perform a little open heart surgery" on the Avengers and "let the audience experience them on a more personal level than they ever have before."

In "Age of Ultron," Marvel's superhero worlds come crashing together as the superheroes tackle a complex villain in Ultron (James Spader), spawned out of artificial intelligence and evolving into a megalomaniac.

With help from twins Pietro (Aaron Taylor-Johnson), known as Quicksilver for his super speed, and Wanda (Elizabeth Olsen), the Scarlet Witch with thought-altering magic powers, Ultron makes the Avengers at times turn on themselves.

Robert Downey Jr., Marvel's Iron Man and patriarch of the franchise, said given the present day "world stage," now is the time for the Avengers to get real.

"This is where you have to actually start dealing with the ramifications of saving the world," he said.

The trend toward somber superh eroes has already proved to be a winner at the box office for other studios. The 2013 Superman film "Man of Steel," from Time Warner Inc's Warner Bros, made more than $660 million worldwide. The studio's "Dark Knight" Batman trilogy grossed more than $2 billion globally between 2005 and 2012.

Olsen said "Age of Ultron" allows audiences to see the "most human versions" of the seemingly invincible Avengers.

"It's darker, more emotional, and more based on the characters as people, as opposed to superheroes," she said.

"Avengers" became the third-highest grossing film of all-time and "Age of Ultron," made for $250 million, is expected to match, and possibly surpass, its predecessor's $1.5 billion take at the global box office.

The strategy to tackle more serious themes may aid the film's box office, said entertainment industry analyst Hal Vogel, "because it seems to reflect a change in the social mood."

Ahead of its U.S. debut on May 1, "Age of Ultron" amassed $201.2 million across 44 countries, outpacing its predecessor's opening weekend take.

"Age of Ultron" shows the superheroes very conscious of the human cost of their war against evil. In one scene where Iron Man and The Hulk obliterate a skyscraper, the aftermath drew comparisons to the very real destruction in Manhattan during the 9/11 attacks.

"It's a very fine line, and that may be a place we crossed it – we were all worried about that," Whedon said. "At the same time we felt it would be disingenuous to say these guys trashed a city and there's no consequences."