London (Reuters) - Elizabeth Olsen and Aaron Taylor-Johnson make their debuts among the world's mightiest in The Avengers sequel, but the two actors said joining a star-studded superheroes line-up was "daunting".

The young actors, both cast in 2014's Godzilla, reunite in The Avengers: Age Of Ultron to play siblings Wanda Maximoff/Scarlet Witch and Pietro Maximoff/Quicksilver.

They star alongside an already established The Avengers cast of Robert Downey Jr (Tony Stark/Iron Man), Scarlett Johansson (Black Widow), Chris Evans (Captain America), Mark Ruffalo (The Hulk) and Chris Hemsworth (Thor).

"It's super surreal and very overwhelming and kind of daunting stepping onto that set, but luckily we'd just done a movie together and I think it helped a lot to have a partner in crime," Taylor-Johnson told Reuters in a promotional interview for the film.

"Besides the personalities being daunting because they're all such amazing actors, it's also daunting to step on and learn about how you're going to portray these characters when it's all second nature to them now," Olsen added.

The Avengers was the fastest film to pass the US$1 billion (S$1.34 billion) mark in box office takings when it was released in 2012.

The sequel sees Tony Stark try to jumpstart a peacekeeping programme but when things go wrong, the superheroes have to stop the villainous Ultron's destructive plans.

The Scarlet Witch is one of the most powerful Marvel comics characters with thought-altering magical powers.

Asked about her portrayal of the character, Olsen said: "I started with just the comic book. You start small and that's two dimensional and you have the script and afterwards you create their back story.

"You start to understand their relationships and sometimes it's when you finish the movie, you're like, 'Oh. Now I understand her.'"