San Diego (CNN) -- The cast and directors of the upcoming "Captain America: The Winter Soldier" appeared this weekend at Comic-Con in San Diego to preview the film and reveal new footage. The movie, scheduled for release on April 4, was directed by brothers Joe and Anthony Russo and is meant to bridge the gap between "The Avengers" and its 2015 sequel.
The footage screened to fans in Hall H reveals the superhero adjusting the modern world and presents Captain America with some new and dangerous challenges compared with the original "Captain America" film, which was set in the 1940s.
"The movie is very different in tone from the first one," Joe Russo said during a news conference after the panel. "The first's one is kind of a love letter to the Captain of that time period. The Captain is now in the modern world and the film is a political thriller. In order to be germane to that tone, we wanted the movie to be modern and edgy and aggressive.
"You can't have thrills in the thriller unless the characters are in a real state of jeopardy. So Captain goes through a lot in this film. It's action-heavy and a very intense movie."
The directors won't reveal too much of the plot, but there are several new characters. Anthony Mackie plays Falcon, an ally of Captain America (Chris Evans).
"Cap is looking for a friend in the modern world," Russo said. "He's sort of lost."
Robert Redford, playing a mysterious S.H.I.E.L.D. agent, appeared in the footage screened on Saturday, and Emily VanCamp was on hand to (sort of) discuss her character, Agent 13.
"I can't reveal too much about the character," VanCamp said. "I think people have a certain idea of what I'm meant to be playing, but I think you'll be surprised at how we use this character. Compared to other characters I've done, it was certainly a challenging experience and an exciting experience."
The central point of the film seems to be a friendship between Captain America and Black Widow (Scarlett Johansson), who initially joined forces in "The Avengers."
"The hurdle of Captain America is that his nature is to put himself last," Evans said. "The first Captain America film is about getting the opportunity to serve, the opportunity to lead, to a soldier. I think he's so determined to do what's right. The conflict with this modern society as opposed to the '40s is in the '40s, it was easy to say 'the Nazis are the bad guys.' Today it's a little bit harder to know 'who am I serving?'
"It's not about doing the right thing, it's about 'what is the right thing?' We get to see Captain struggling with that. That's where his relationship with Black Widow comes to fruition. She's someone who has had to compromise her morals and not live in black and white, and I think as a result -- I can't say too much -- something happens. These two people from different worlds need to rely on each other."
Sebastian Stan plays the titular Winter Soldier, an altered version of his character Bucky Barnes from the first "Captain America" film. Here Stan is now the villain. The actor said the character deals with similar challenges as his foe in dealing with the modern world.
"In a sense, he's very similar to (Captain America)," Stan said. "He's another lost man in this new world. As we know, the journey for him is then just discovering his own identity and his role in this new world, in his own way."