[Editor's note: The following contains spoilers for The Haunting of Bly Manor.]

Part of the appeal of Netflix's The Haunting of Hill House was that it's a show you watch twice  — once for the deeply moving horror story about trauma and family, and a second time just to find the ghosts that showrunner Mike Flanagan and Co. sprinkled in the background throughout the season, hidden in the shadows. If you've made it through Hill House's follow-up, The Haunting of Bly Manor, you might have noticed the same deal underneath all that tragic Gothic romance; a plague doctor or a doll-faced child lurking just out of focus. But when Collider's Drew Taylor sat down with Flanagan and producer Trevor Macy, the creative team revealed that Bly Manor's background ghosts served a much more specific purpose than a fun, spooky Easter Egg hunt.

Here's what Flanagan told us:

"In the first season, it was really just kind of a little spice we wanted to put on it. We didn't really want to do anything with it other than kind of create that kind of re-watchability and hopefully just spook out a couple of people. We executed it by having extras dressed and in makeup every day standing by, usually at the crafty table just kind of eating candy and waiting for every shot to go up. We would look for an opportunity to throw them in.

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It would be as fast as after we finish our first team rehearsal, flying and go. See if we can find a spot. This season though we didn't want to do that again. For Bly, it was important that if we could find a way that the ghosts could actually be part of the narrative this time instead of so many random faces, if we could kind of cast them, which is a tough sell for a performer to say we want you to play a part where hopefully we never notice you and you hopefully not seen until the end, but I wanted it to really fit into the story and that we'd find out eventually who they were."

The fun of the show's hidden ghosts is that they're really hidden, occasionally only just visible enough to count as on-screen. The cast wasn't immune to the ghost hunt, either. Flanagan told us that the actors didn't always know when a spirit was lurking in their scene.

"I would always try not to tell the cast if there was going to be a ghost in the shot. I like to mess with them that way. Sometimes they don't notice until their take is over and they turn around and someone's in the fireplace. Occasionally, we forget about them. We bury them so deep in the background, it happened to the plague doctor all the time on the exteriors. We'd place him by the lake way deep and we'd be frantic to kind of keep our day going. We'd move on and realize we'd left him out there in the water. Patiently waiting for us to tell him to move."

For more on The Haunting of Bly Manor, here is our full review and a deep-dive explainer for this season's Lady in the Lake.

And for more from our interview with Flanagan, click on the links below. Look for the full interview on Collider soon.

• Mike Flanagan on the Heartbreaking 'Bly Manor' Finale and If That's Dani's Hand
• 'The Haunting of Bly Manor' Dream Hopping Explained by Showrunner
• How a Scrapped 'Hill House' Idea Inspired 'Bly Manor's Flashback Episode 8