Though we already knew that Peggy Carter would make at least one appearance in the first animated series set in the Marvel Cinematic Universe, we didn't know for certain that Hayley Atwell would return to give her life. Fans can rest easy now, though, because What If? will definitely include Atwell.
Atwell confirmed her participation in an interview with The Hollywood Reporter, and also remarked on the short shoot for her brief (but highly memorable) appearances in Avengers: Endgame. She got the call about Endgame mere weeks before she filmed it.
"I think we shot that maybe two years ago," said Atwell. "So, it was maybe a few weeks before then that I got the call saying, 'We’re thinking about this… What do you think about it? It might end the whole franchise.' Then, it was an afternoon of filming about two years ago."
Her reaction to Peggy Carter, beloved by so many fans, literally being the endgame of Endgame was very positive. "I thought it was a fitting end to a story that has affected so many people. I thought it was very endearing, innocent and wholesome in the way that it keeps those characters in their time. I thought it was quite beautiful and very tasteful of Marvel to finish this 10-year story in a very simple storyline about two human beings — and one of them doesn’t even have any superpowers," said Atwell. "So, I thought the tone of it, to end there, after some extraordinary things of trauma, action, effects and powers… to just have two people slow-dancing was very beautiful."
In case fans are hoping to see more of Atwell's Carter in deleted Endgame scenes, the bad news is that there aren't any. According to her, everything she shot is in the movie: "You see me through the office blinds, and then that scene with the slow-dancing. Those were the two that I was always intended to film."
However, where fans can soon look to see more of Peggy is in What If?, which was officially announced at San Diego Comic-Con 2019, where she'll supposedly play Carter becoming a super soldier in World War II instead of Steve Rogers. This animated appearance aside, Atwell seems to be at peace with ending her run as Carter, saying that she feels "really fulfilled." Though fans keep hoping for an Agent Carter revival, that might not be in the cards anymore. Atwell is currently returning to her roots in classical theater, having just finished a run of Ibsen's Rosmersholm in London's West End.
As she says, "It was a great time, and I think they ended it beautifully with Endgame. It feels like a fitting end to that narrative."