Marvel is notoriously secretive with scripts, for understandable reasons, so the actors themselves sometimes don’t know what’s going on with their characters next. This has certainly been the case with Anthony Mackie, who was introduced as Falcon (Sam Wilson) in Captain America: The Winter Soldier and didn’t even know he’d be in The Avengers: Age of Ultron until he saw himself on a poster. He filmed two scenes for the film afterward, but only received the bare minimum amount of script needed for those scenes. Which means — as Mackie revealed to IGN on the set of Captain America: Civil War shortly after Age of Ultron opened in theaters — he didn’t even realize Falcon had joined The Avengers Initiative until he sat down at the Ultron premiere and watched the film with his co-stars.
“When we were at the premiere I was like ‘Cool, I’m with my son and he gets to see his daddy in two scenes in The Avengers.’ By the end of the movie I turn around and they’re like ‘Avengers Assemble!’ I’m like, ‘Holy sh*t! I’m an Avenger!'”
As he explains, he never heard the part about Avengers assembling when he filmed the scene. He was there only during his fly-in stunt, explaining to IGN, “Six months later, you go to a premiere and you sh*t yourself.”