"The Iron Ceiling" shows that Dottie is 19 years old, 20 at the most. She is played by Bridget Regan, who was 32 years old during filming.
Hayley Atwell and Dominic Cooper were, respectively, 32 and 36 years old during the filming of Season One. Their characters ages in 1946 are 27 and 29.
Dyeing for Your Art: Bridget Regan is a natural redhead, but according to an interview she dyed her hair blonde to play Dottie because the producers were afraid that Dottie being a redhead would give away the fact that the character is a proto-Black Widow. They also didn't want the audience to think that all Black Widows are redheads.
Enforced Method Acting: Sousa's crutch was made to be slightly lower than was comfortable for Enver Gjokaj - so he would lean on it a bit more.
As you can tell by the actor's name, Ralph Brown, who plays Dr. Ivchenko, isn't Russian. He's actually British.
Mildly with Hayley Atwell, who's actually half-American-half-British while Peggy is full-blooded British.
American actor Jack Conley portrays Nazi German Ernst Mueller.
American actress Bridget Regan plays a Russian character who poses as an American, exactly like Scarlett Johansson's Black Widow. However, the show provides an explanation for why they both speak in an American accent: they are indoctrinated through subliminal messages hidden within American cartoons of which they must replicate the voices and accents within..
Toby Jones returns in the season 1 finale, once again an Englishman playing a Swiss man.
Lotte Verbeek is Dutch, and she plays the Jewish Hungarian-born Ana Jarvis. She mostly tones down the American pronunciations in her accent, but doesn't sound Hungarian to ears that know the difference.
Fan Nickname: Agent One Tree Hill for Jack Thompson due to it being his most famous role. Agent Chad Michael Murray is also commonly used as a nickname for him, since that's the name of his actor. And, in the right corners of fandom, you will occasionally see him called Agent Douchebag, for his general bad attitude during the series.
I Knew It!: Dottie being a lethal assassin. Some viewers were calling it as far back as the announcement that Bridget Regan had been cast for the role, while others figured it out in the scene that introduced her as an aspiring ballerina because they immediately made the connection to the Black Widow.
The Other Darrin: Sort of. Though Tony Stark's JARVIS AI presumably has his voice modelled on the real Edwin Jarvis (the personalities certainly match up to a "T"), James D'Arcy takes on the role of the human original, rather than having Paul Bettany play both incarnations of the character.
James D'Arcy was initially nervous about playing the kind and comical Jarvis, as during most of his career he's played psychopaths. Retroactively, Edwin Jarvis' fans might be going to be shocked that many of D'Arcy's previous roles are so unlike Jarvis.
Ken Marino typically plays silly and comic characters of some sort, whereas Joseph Manfredi is a gangster who is able to indulge in violence if he feels necessary.
Production Posse: Ken Marino (Joseph Manfredi) previously worked with Tara Butters and Michele Fazekas in Reaper.
Role Reprise: In the Latin American Spanish dub, most of the voice cast from the Marvel Cinematic Universe films reprised their roles. This is especially relevant in this case, since the dub of the both the TV series and the films was done in Mexico, when traditionally the dubs from Marvel's TV series for Netflix are normally done in South America, mainly Argentina and Chile, albeit in this case this is because the dub was funded by Sony, and not Netflix, so they didn't have a choice on this, and mostly for continuity reasons.
Trolling Creator: Jack Thompson was teased as a candidate for Peggy's husband in the marketing for the first season, despite the fans' hatred of him. There are never any hints towards this in the show itself, and there probably will never be after he was shot at the end of Season 2.
Angie was originally going to appear in Episode 5, but due to contract legalities Lyndsy couldn't appear, so an excuse was written that Angie was "too consumed by ennui".
In the season finale climax, it was originally going to be Dum-Dum Dugan on standby to shoot down Howard's plane, while Jarvis would be the one trying to talk him down (as a Call-Forward to J.A.R.V.I.S. and Tony's chats while in flight.)
Hayley Atwell talked about the possibilities of other seasons doing Time Skips focusing on different eras in Peggy's life - for example what she was doing in the 50s or 60s. The Crown & The Flame would use a similar idea - with the third season skipping forward two decades and recasting everyone with older versions of their characters.
James D'Arcy almost didn't take the role of Jarvis due to his commitments to Broadchurch. Literally his last day of shooting on the latter had to wrap at midday, and he had to board a plane to Los Angeles two hours later.
Word of God: The writers have made several statements concerning the future of the show, as well as how it connects to the Marvel Cinematic Universe, such as the connection between Jarvis the butler and JARVIS the AI, as well as Dottie's origins.
You Look Familiar: Enver Gjokaj, who plays Daniel Sousa, played one of the police officers Captain America gives instructions to in The Avengers. The producers joked that Sousa was the cop's grandfather.