Flanagan said that every episode features hidden ghosts and that actors were "on standby" for every episode.
For instance, when the young Hugh Crain (played by Henry Thomas) is running through the house carrying the young Steven (Paxton Singleton) in episode one, there are ghosts hidden throughout the house, in the shadows and in the background, that are hard to spot unless the viewer watches it again.
"We would hide ghosts in plain sight in every episode," Flanagan said. "We have dozens of them in every episode ... that was one of my favorite ideas about the original pitch that I thought would be really fun."
"Ghosts" were on set every day of shooting the first season, but there was no guarantee they'd actually be working on any given day.
"We would call [the actors] early in the morning and they would go through makeup and be on standby, basically hanging out at craft service scaring anyone who went to get candy bars," Flanagan said.
"We'd spend 15 or 20 minutes before camera rehearsals trying to find a place to hide our ghosts," Flanagan added. "We did that every day. Sometimes it would work, but the big test was whether we could see them clearly if we were looking but would blend in. Sometimes we would look at them and think it was too obvious or too subtle and we wouldn't use them. But every single day that we rolled cameras at the Hill House, we had ghosts standing by that were ready."
Flanagan said the same people were used "again and again" because "not many people are willing to do that much makeup and sit around all day on the off chance that they'd be used in a shot in which the audience is not meant to see them. That's a tough sell for an actor."
"However many [ghosts] you think you found, whether it's true or not, I will always say there's one more," Flanagan said.