The 32-year-old actress will reprise her role as Peggy Carter in the movie. The character is Captain America's WWII love interest and a S.H.I.E.L.D. agent, and appeared in Captain America: The First Avenger (2011) and Captain America: The Winter Soldier (2014).
Entertainment Weekly visited the Age of Ultron set for a Comic-Con double issue, and Atwell told the magazine her inclusion in the film came about during a night on the town with director Joss Whedon.
"We went out drinking one night and he was like, 'You know, if would be great to write something for you,'" she shared. "And I was like, 'Yes, Joss, it would! It would be amazing!'"
The context of Atwell's cameo is unknown, but some fans speculate her character will appear in flashback sequences. Carter and Captain America met and fell in love in the 1940s, but she believed him dead after he crashed in the Arctic Ocean and disappeared at the end of The First Avenger. She became a S.H.I.E.L.D. agent after his disappearance, and appeared in her old age in The Winter Soldier. The poignant scene sees an unaged Captain America visit Carter in a nursing home before her death.
In addition to her Age of Ultron cameo, Atwell will reprise the character on upcoming television series Agent Carter. The program premieres on ABC in January, and focuses on Carter's life as a S.H.I.E.L.D agent.
Avengers: Age of Ultron is scheduled for release May 1, 2015.