(at around 37 mins) Hayley Atwell (Peggy Carter) surprisingly touching Chris Evans' chest, as he emerged from the pod upon turning into Captain America, was very much improvised, and the surprise on her face is genuine, as she admitted in interviews she was very taken by Chris' physique and nearly broke character and ruined the take that made it into the film as a result.
Most of the shots were done by a Los Angeles company called LOLA, that specializes in digital "plastic surgery". The technique involved shrinking Chris Evans in all dimensions. They shot each "skinny Steve" scene at least four times. Once like a normal scene with Evans and his fellow actors and actresses in the scene, once with Evans alone in front of a greenscreen, so his element could be reduced digitally, again with everyone in the scene but with Evans absent, so that the shrunken Steve could be re-inserted into the scene, and finally, with a body double mimicking Evans' actions in case the second technique was required. When Evans had to interact with other characters in the scene, they had to either lower him or raise the other actors and actresses on apple boxes, or elevated walkways to make "skinny Steve" shorter in comparison. For close-ups, Evans' fellow actors and actresses had to look at marks on his chin that represented where his eyes would be after the shrinking process, and Evans had to look at marks on the tops of the actor's head to represent their eyes. The second technique involved grafting Evans' head onto the body double. This technique was used mostly when Evans was sitting or lying down, or when a minimum of physical acting was required.
Chris Evans declined the role three times before accepting the part. Not out of dislike for the role, but because he feared what the effects of the sudden increase of fame would be on his private life. Then Robert Downey Jr. convinced him to take the part, and thus gain the freedom to sign on any other role he'd want afterwards. After that, he had a meeting with director Joe Johnston and the producers, who convinced him to take the role.
Originally, cameo appearances were planned in the film for James "Logan" Howlett (Wolverine) and Erik Lehnsherr (Magneto), who were present during World War II (Logan was a soldier and Lehnsherr was in an internment camp). These cameos were scrapped due to rights issues. Marvel Studios now owns the rights of the X-Men characters, a result of the Disney-Fox merger of 2019.
Stanley Tucci took the role of Dr. Erskine because the role enabled him to use a German accent, which he always wanted to do.
Stan Lee: (at around 1h 12 mins) Military dignitary at a medal ceremony for Captain America. He is also the older gentleman saying: "I thought he'd be taller", when Captain America fails to show up at Senator Brandt's ceremony. This is Lee's only cameo in a feature film adaptation of a Marvel Comics superhero character where he was not an original creator. However, he did create the element of Captain America using his shield as a returning throwing weapon in an early text story in Captain America Comics #3, "Captain America Foils the Traitor's Revenge". Furthermore, he was responsible for reviving the character in the 1960s, first as a member of The Avengers, and then in a solo series which focused on the hero being a troubled man out of his time after years of being in suspended animation. Except for Deadpool, Guardians of the Galaxy, & Captain Marvel,