In the beginning of "Avengers: Endgame" is a montage of scenes showing Nebula and Tony Stark, trying to get back to safety on the damaged Benatar. It turns out that most of those scenes were improvised.
Karen Gillan, who played Nebula, explained in an interview that the scenes between her and Downey Jr. did not have dialogue. The script only stated that she and Downey Jr.'s characters play football, eat, and try to pass the time while lost in space, and so, both actors decided to flesh the moment out with their own repartée.
Gillan also revealed the improv happened because of Downey Jr., who she credits as "an amazing improviser." She went on to say, "And so there was all this stuff about them playing the game and how much it meant to Nebula that she got to win something.”