It seems each portion of the Marvel panel had a big ensemble cast present on the stage (aside from Tom Hiddleston, because, the man can seriously play the crowd all by his lonesome with ease and grace), and Captain America: The Winter Soldier was part of this theme.
Producer Kevin Feige brought director Anthony Russo and Joe Russo out before introducing the cast, which was Chris Evans (Steve Rogers/Captain America), Sebastian Stan (Bucky Barnes/Winter Soldier), Anthony Mackie (Sam Wilson/The Falcon), Scarlett Johansson (Natasha Romanoff/Black Widow), Frank Grillo (Brock Rumlow/Crossbones), Emily Van Camp (Agent 13), Cobie Smulders (Agent Maria Hill), and Samuel L. Jackson (Nick Fury).
Yes, Chris Hardwick, the best moderator there ever was, photobombed again.