It's currently unknown what character Serkis is playing.
Serkis is known for his motion capture work in The Hobbit and Planet of the Apes. He also helped on The Avengers 2 with the Hulk and will be featured in Star Wars: Episode VII.
Updated: with official version:
"The Avengers: Age Of Ultron" hits May 1, 2015 directed by Joss Whedon starring Robert Downey Jr., Chris Evans, Chris Hemsworth, Scarlett Johansson, Mark Ruffalo, Jeremy Renner, Samuel L. Jackson, Elizabeth Olsen as Scarlet Witch, Aaron Taylor-Johnson as Quicksilver, Paul Bettany as Vision, with James Spader as Ultron.
Head on over to the Cosmic Book News Avengers movie hub for more.