Robert Kurtzman reveals that up to 30 "Easter egg" ghosts are planted throughout the series. "It was [creator/director Mike Flanagan's] plan from the beginning to set up these kinds of little Easter eggs and see if people could spot them." While working on the primary prosthetic makeup for the show each day, the makeup team would also produce up to four of the "hidden ghosts" as well for Flanagan to pepper in the shadows.
Mckenna Grace (Young Theo) and Kate Siegel (Older Theo) both kept journals in the persona of their character, writing about Theo's experiences and thoughts. The actresses would then swap journals with each other to gain a better understanding of the younger and older version of the character respectively.
The Crain children represent the five stages of grief: Denial (Steven), Anger (Shirley), Bargaining (Theo), Depression (Luke), and Acceptance (Nell).
In multiple wide-shot scenes at Hill House, you can see ghosts in the background, sometimes just hands, shadows, and faces.
Director Mike Flanagan has repeatedly stated that making this series was the worst and most stressful professional experience of his career. The creator/director lost 40 lbs. during the show's six-month production.