After three weeks floating in space with no hope of being rescued following the catastrophic end of Avengers: Infinity War, Tony and Nebula were safely brought back home thanks to Captain Marvel, reuniting him with his fellow surviving Avengers. Unlike the Earth-bound heroes who were actively looking for the Mad Titan in the hopes of undoing The Decimation, Iron Man was physically and emotionally spent. So when Captain America tried to squeeze him for any information that could help locate the villain, Tony snapped. Lashing out, he reminded the super soldier how his repeated warnings about this imminent threat fell on deaf ears, and how Steve valued his own freedom over the safety of Earth in Captain America: Civil War. He also pointed out how Captain America promised him that when push came to shove, they would deal with this extraterrestrial danger together, and if they lost, they would do that together too. Since Tony battled and lost to Thanos while alone on an alien planet, Tony ended his impassioned speech by calling Steve a “liar” to his face. It was undoubtedly one of Downey’s best-acted scenes in the MCU, and it turns out, he improvised most of it.
“I think that was one of Downey’s most inspired performance moments in the movie. He very much reverts to, this is the guy who felt forsaken by his father - you can see his intimacy and trust issues in that moment when he turns on Cap. Downey performed the scene with a lot of energy. We didn’t do it many times, because he was expending himself so, so much. He understood it very well.” (x)