Benedict Cumberbatch may not be singing his own praises, but were lucky Scott Derrickson took to social media to share the improvisation with us. There are actually a lot more moments like this in Endgame then you might expect. Karen Gillan totally improvised throughout in her scenes with RDJ playing paper football, and Downey had a number of his own lines, including the “liar” moment in the beginning, and his final scene went through many iterations before it made us sob on the big screen.
Article source: https://www.cinemablend.com/news/2557972/one-of-avengers-endgames-most-iconic-moments-was-improvised