Among the large ensemble cast assembled for FX’s “American Horror Story’s” sixth season was a rookie – one that would appear in 85% of the finished series: Roanoke House.

The 8,000-sq.-ft. built-from-scratch structure – reflecting gothic, colonial, art nouveau, and Shaker architecture – was production designer Andrew Murdock’s crowning achievement, and a star of the show.

Constructing such a focal point is often a production designer’s most critical job – a task that can be as key as proper casting or solid writing.

But more often a key set is about developing the walking, talking characters around it. In the case of Netflix’s “The OA,” production designer Alex DiGerlando was tasked with creating a different sort of housing structure — glass cubes that would hold five prisoners in an underground cave — that would have a far-reaching influence on the series’ title character, played by show creator Brit Marling.

DiGerlando knew he had to get the cubes right because they “informed so much of the OA’s character,” he says. “There’s so much passage of time spent in there that it’s like this living, breathing, evolving set. It’s where the OA is born; it’s where Prairie becomes who she is ultimately going to be.”

John Paino’s Blues Café in HBO’s “Big Little Lies” is a pier-side coffee shop, made of weather-beaten wood. It was custom-built and featured multiple windows to give it an airy, yet cozy feeling, and was meant to draw together women from both sides of Monterey’s tracks.

“It’s a sanctuary for the gals to let their hair down, like a no-man safe zone,” Paino says. “It was a central meeting place for the story, and we had to make it feel open and inviting, a place where women could walk in any time — and where Jane [Shailene Woodley], not as affluent as the other women, could feel comfortable.”

One of the hallmarks of a great set is that it supports the characters and doesn’t overwhelm them. “The house had to exist on its own terms,” says Murdock. “The story has to come first.”