Basically, the cast didn't know the context of their scenes until they arrived on set.
Sebastian said one scene in particular that was kept a secret was the fact that everyone was gathering for Tony's funeral.
He said that the cast was told that they were filming a wedding by a lake and his costume consisted of nice attire.
On the day of filming, he arrived and found out who was in the scene...and who wasn't.
It became clear very quickly that it wasn't a wedding, but rather Tony's funeral, and something had happened to Natasha.
Sebastian talked generally about the moment so he wouldn't spoil Endgame live on TV, and joked that it was TOTALLY a wedding.
Basically, the cast who played the dusted characters learned Tony and Natasha didn't survive while the cameras were rolling and now I'm even more emotional.
So, whatever you do, don't think about how the scene consists of a lot of the cast's genuine reactions to the news. Don't do it or you'll be crying, like me.
