Theron, who announced the news she had adopted a baby boy earlier this week (beg12Mar12), and the Irish actor met fans and the media and chatted about their much-anticipated new film, and revealed the event gave them the chance to see clips from the film for the first time.
Theron said, "It's nerve-wracking for us because we haven't seen anything, so we're actually seeing what they're (fans) seeing for the first time too."
Fassbenber admitted he was thrilled to be at the convention: "Anything in the comic book fantasy world it breeds a very passionate fan base."
The film's director Ridley Scott, Milla Jovovich, Emma Stone and Joseph Gordon-Levitt also attended WonderCon on Saturday.
Theron also joined her Snow White & The Huntsman co-star Kristen Stewart for a panel chat about that film too.
Asked by one fan why she likes to take on dark film roles, Theron answered, "It's because I'm a b**ch!"
The actress plays the evil queen in the fairytale adaptation.