We spent a lot of time in the edit room playing around with the sequencing of the portals. We probably didn’t lock that section of the movie until about a month before the film was in theaters. We always wanted Sam to be the first one to communicate with Cap via his comm, and Sam was last in Wakanda, so logically the first portal that would open would the Wakandan portal. And the first person that would logically walk through a portal from Wakanda would be the king himself, bringing his army once again to the defence of Cap and the world. – Joe Russo

