PARIS — French model-turned-thesp Laetitia Casta (“Gainsbourg”) will topline “Behind the Walls,” a 3D low-budget suspenser directed by Pascal Sid and Julien Lacombe.

The 1922-set fantasy thriller turns on a young novelist who moves to a creepy village to write a book and falls victim to terrifying hallucinations and nightmares.

“Walls” is produced by Paris-based Sombrero Films’ Alain Benguigui and Thomas Verhaeghe, via their new label Studio Mad, which is dedicated to genre films.

The $5 million pic will be distribbed by Bac Films during the first trimester of 2011.

Roissy Films will handle international sales.

French paybox Canal Plus, Orange Cinema Series and CineCinema have pre-bought TV rights.