Trip Cullman directs the comedy, which centers on Porter (Grace), a married-and-divorced "corporate ninja," recovering from a nervous breakdown, who meets an ambitious businesswoman (Thirlby) with her own emotional obstacles.
Grace's credits include the films In Good Company and Spider-Man 3, as well as a seven-year run as Eric Forman on the television series That '70's Show. Thirlby made her Off-Broadway debut in Beau Willimon's Farragut North and has been seen in films including Juno and No Strings Attached.
Weitz's plays include Trust, Show People, and Privilege, as well as the films About a Boy and Antz, among others.
Lonely, I'm Not will feature scenic design by Mark Wendland, costume design by Emily Rebholz, lighting design by Matt Frey, sound design by Bart Fasbender, and projection design by Aaron Rhyne.
Additional casting will be announced shortly.