When T'Challa was one of those done and dusted at the end of Avengers: Infinity War, it was a gut punch. This was a hero who had literally just been introduced to the pantheon. Black Panther was such a hit, it was just finally departing theaters as Infinity War rolled in. But the Russo Brothers made their movie at the same time Black Panther was filming, so they had no idea how big it would be. But it also provided opportunities, like when the Black Panther cast improvised an Infinity War scene, one of the most affecting of the entire film.
One of the reasons Black Panther and Infinity War filmed simultaneously was due to the heavy use of Wakanda in Infinity War's final segment. Since they were using the same group of characters, and many of the same extras, it was easier to have them migrate from one set to the other.
But it also meant the Russos were making Infinity War without knowing anything about Black Panther. They had no idea what was happening other than the costume and set crossovers. They didn't see Ryan Coogler's finished dailies, and they didn't know much about what kind of world he was building over there.
So, when they arrived on the set one morning to start the long process of filming the battle scene against Thanos' minions, they were not prepared for their Wakandan cast to be ready and waiting, and with their own notions of exactly how their country goes to war.
Director Joe Russo says in the Blu-ray that when they rolled and M'Baku began leading the entire cast of extras in a call and response chant, they were stunned and delighted.
We did not know — because we had not seen Black Panther because they were still making Black Panther — about their war chants. And they just started doing it.
Anthony Russo interjects: "Oh, my God, yeah."
The two directors immediately understood their actors had a better hold on what they could and should do here to tie the two films together. So they let Winston Duke, who plays M'Baku, direct the chanting, and let the improv go from there, which lead to the second call and response with T'Challa that followed.
After watching Winston doing it in the opening of the sequence, it led to the responding war cry that T'Challa (Chadwick Boseman) leads.
It was a scene so affecting, it actually became its own trailer.
Without the actors, the Russos would have never known to do any of this. Joe Russo says in the Blu-ray:
A year ahead of time, we had no idea 'Wakanda forever!' was going to be amazing.
Even though Infinity War did manage to tie itself to Black Panther as best it could sight unseen, there are places where the Russos were having to work in a vacuum shows. They turn up in small ways, like not showing if Shuri was dusted or not when Thanos snapped his fingers. Marketing has wisely turned it into a cliffhanger, but in reality, they didn't think to show her because no one on set knew she would be such a breakout character.
Meanwhile, Avengers 4: It Might Be Called End Game, But We'll See arrives in theaters on May 3, 2019.