(at around 46 mins) According to writer Eric Pearson, Florence Pugh teased Scarlett Johansson about Black Widow's iconic pose during filming, so Pearson put it in the film.
(at around 53 mins) In prison, Red Guardian shows a tattoo on his right shoulder of two roses. The two Russian names underneath them translate to Natasha and Yelena.
Ever Anderson secured the role as young Natasha because she already speaks Russian and has a green belt in taekwondo.
(at around 1h 22 mins) Yelena mistakenly calls Alexei "Crimson Dynamo" rather than Red Guardian. In the comics, Crimson Dynamo is a Russian supervillain in a powerful armor who usually fights Iron Man, and has been a teammate of Red Guardian. His real name was Anton Vanko, and he is the father of Ivan Vanko (Mickey Rourke) from Iron Man 2 (2010), whose electricity-based weaponry and armor were based on the Crimson Dynamo from the comics.
Black Widow's wrist-mounted tasers, commonly referred to as her Widow bites or gauntlets, for the first time are painted gold. This is an homage to Natasha's classic Black Widow costume from the comics, which was an all-black suit with a gold ringed belt and gold gauntlets.