Frank Spotnitz gets interviewed by TV Fanatic about the show, and talks about how shifting historiography affected his adaptation:
TVF: Did you have to learn more about World War II and just all the elements that went into that because this takes like a different step than, of course, what history actually did?
FS: Well, yes. It occurred to me and [Philip K. Dick is] an absolutely brilliant guy, but he wrote the book in ’62 and our understanding of the war would be different at that point than it is today, so I called three different historians who specialize in World War II. I asked them a bunch of questions and a lot of the changes that I made from the scenario that he wrote down in the book were formed by my discussions with the historians.
I'd love to know who he called in and what they said.
And the cultural details were given considerable thought:
TVF: Tell me how the look of the show came to be because it’s beautiful to look and has its own unique style from the start.
FS: I have a fantastic director, David Semel, and production designer, Drew Boughton. We had a lot of talks about how things would have unfolded if we had lost. What cars would people drive and what clothes would they wear? We thought caps would probably be more popular because JFK would never have been president and he is one of the people who sort of made the caps less popular because he didn’t wear them.
We thought military uniforms would actually change very slowly, so they wouldn't much different. We spent a lot of time on things like the flags for the Nazi state and the Pacific states and the Japanese states and all kinds of things. We were just trying to get every detail right because we were obviously hoping it would become a TV series and we knew that the decisions you make in the pilot you have to honor the rest of the show, so we didn’t want to make any mistakes.
TVF: You even come up with fake movie titles and game shows, I did Google [the movie title] The Punch Party because I didn’t remember that film. It didn’t exist but was so perfect that it was Rock Hudson and June Allyson, but even that was a nice touch.
FS: The most popular comedy in Nazi Germany was called The Punch Bowl, so I thought well maybe there was a sequel, you know for the American territory. Rock Hudson and June Allyson are appropriately wholesome stars who you could imagine would be approved by the Nazi’s but obviously Rock Hudson has a secret and I thought it just tells you so much already from the get go.
I think the show still underestimates how different America would have been in such a (highly implusible, admittedly) world. But within their premises, it's a compelling and consistent vision.
And: multiple seasons are planned. Yes, multiple seasons of an alternate history TV show. Time to get excited. Amazon says it was their most watched pilot of all time.