The medical drama focuses on a socially awkward new doctor called Emily who discovers that life at the hospital she joins is no different from her high school.
Rady has now joined the cast as surgical resident Micah, TV Line says.
Meanwhile, McCreary will play Emily's fellow medical intern Tyra.
Rady's previous credits include Swingtown, Greek, Melrose Place and The Mentalist, while McCreary has appeared on shows like Rubicon and White Collar.
First Cut has been penned by Jennie Snyder Urman, who has written for Gilmore Girls and 90210.
Other pilots currently in development at The CW include superhero drama Arrow, Sex and the City prequel The Carrie Diaries and crime thriller Cult.