Two productions from Roundabout Theatre Company explore scary demons just in time for the Halloween season. The big star vehicle, Thérèse Raquin, is full of fake emotion while the Off-Broadway intimate drama, The Humans, is truly terrifying in its portrayal of the bumps and creaks in the night we all hear and fear.

The first act of Helen Edmundson’s stage adaptation of Emile Zola’s classic 1867 novel Thérèse Raquin from Roundabout at Studio 54 really had me going. I was totally enraptured by Keira Knightley’s nearly silent performance as the titular frustrated heroine, expressing her sexual and spiritual longing through body language and eloquent features. Thérèse is trapped in a passionless marriage to her bourgeois cousin Camille, first in a provincial backwater and then in a confining Paris apartment. Edmundson’s conceit is that Thérèse can only react to the stifling conditions of her life and remains silent as the oafish Camille and his control-freak mother order her existence. That is, until Camille’s dashing friend Laurent, a would-be painter, enters the picture. (Spoiler alert here if you have not read the novel or seen any of the numerous previous stage versions including Harry Connick Jr.’s 2001 musical update Thou Shalt Not.) The connection between Thérèse and Laurent is electric, and they plot to eliminate Camille. The drowning scene on a real river is really scary; kudos to director Evan Cabnet and set designer Beowulf Boritt.

So far, so good, but in the second act Thérèse opens her mouth. Knightley and Matt Ryan as Laurent start overacting all over the place and Cabnet turns a tragic tale of passion into an episode of Dark Shadows. The lovers become racked with guilt and imagine Camille’s accusing ghost haunting them as Josh Schmidt’s twisted sound design and Keith Parham’s haunted-house lighting grow more ominous. There are some effective moments, mostly provided by Boritt’s impressive set. Thérèse seems to be crushed by her all-black apartment as it descends from the flies and she appears to soar when she meets Laurent in his attic, suspended above the stage amid a starry backdrop (Parham’s lighting achieves the right romantic tone here.) Gabriel Ebert’s comically clueless Camille, Judith Light’s well-meaning Madame Raquin, and Jeff Still, David Patrick Kelly and Mary Wiseman as a trio of shallow family friends provide welcome depth. But they cannot rescue this screamfest from the spookhouse.

Thérèse attempts to evoke genuine fear, but The Humans succeeds in doing so. Stephen Karam’s new play starts out like a dozen other dysfunctional-family works, with the Blake clan revealing harsh secrets on Thanksgiving as the turkey is served and the wine flows. What sets this haunting and heartbreaking drama apart is the subtle depiction of the nightmares which invade and twist the lives of everyday people. The six characters’ fears for the future take various frighteningly familiar forms. Dad Erik obsesses over terrorist attacks and floods. Mother Deirdre forwards emails of dire scientific studies to her daughters Aimee, a lawyer struggling with losing her lesbian lover, her job and her health, and Brigid, a young composer facing a dead-end career. The senile grandmother Fiona (“Momo”) is lost to dementia and Richard, Brigid’s much older boyfriend, has recovered from depression but still has bizarre dreams, but much less frightening than Erik’s, which involve a faceless woman and a forbidding tunnel.

During 90 intermissionless minutes, an expert cast, directed with subtlety by Joe Mantello, convey the petty conflicts and major tragedies of these frightened people, beset by the shifting and uncertain landscape of modern America. Lights switch off, weird sounds emanate from all over Brigid and Richard’s spacious but crumbling Chinatown duplex (great set by David Zinn and sound by Fitz Patton), and the lives of the Blakes are gradually revealed as pitiful and desperate. The entire cast is top-rate, with veterans Reed Birney and Jayne Houdyshell delivering their customary solid work, but Cassie Beck’s Aimee is outstanding in this stand-out ensemble. Her shattered, scattered cellphone call to an estranged girlfriend is a heartbreaking moment in an intensely real performance.

Just after it opened, The Humans announced its transfer to Broadway next year. It will be fascinating to see if this disturbing, unflinching look at the way we live now succeeds on the Great White Way.

Thérèse Raquin: Oct. 29—Jan. 3, 2016. Roundabout Theatre Company at Studio 54, 254 W. 54th St., NYC. Tue.—Sat., 8 p.m.; Wed., Sat., Sun., 2 p.m. Running time: two hours and 30 mins. including intermission; $47—$137; (212) 719-1300 or www.roundabouttheatre.org.

The Humans: Oct. 25—Dec. 27 (then moving to Broadway with dates and theater TBA). Roundabout Theatre Company at the Laura Pels Theatre, Harold and Miriam Sternberg Center for the Theatre, 111 W. 46th St., NYC. Tue.—Sat., 7:30 p.m.; Wed., Sat., Sun., 2 p.m. Running time: 90 minutes with no intermission; $99; (212) 719-1300 or www.roundabouttheatre.org.