There were a lot of really awesome moments in Avengers: Infinity War but one of them turned out to be completely improvised. Directors Anthony and Joe Russo talk about the scene in Wakanda when the war chants started.
According to the Russo Brothers in the director's commentary on the newly home-released Avengers: Infinity War Blu-ray/DVD, the chant wasn't in the original script:
Joe: "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: "Oh, my God, yeah."
It became such a moving moment that it was given it's own trailer in the marketing lead up to Infinity War. The Russo's were filming Infinity War while Black Panther was being worked on so they didn't know what they had initially.
Joe: "And after watching Winston doing it in the opening of the sequence, it led to the responding war cry that T'Challa (Chadwick Boseman) leads. A year ahead of time, we had no idea 'Wakanda forever!' was going to be amazing."
We all knew that Black Panther was going to be a big deal and the fact that we got that fantastic scene because the cast saw a good opportunity to improvise says a lot.
Summary: As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment – the fate of Earth and existence itself has never been more uncertain.
Avengers: Infinity War, directed by Anthony and Joe Russo, stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Chadwick Boseman, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Don Cheadle, Tom Holland, Bradley Cooper, Sean Gunn, and Anthony Mackie.
It's out now on digital and out on DVD and Blu-ray on August 14th.