The comedy focuses on the friendship between two architects - Charlie is straight, while Louis is gay.
Routh has now signed up to play Louis's partner, a former alcoholic who is now a nurse, Deadline reports.
Routh previously played Superman in Superman Returns and appeared in Scott Pilgrim vs The World. He also starred as Daniel Shaw in Chuck.
Meanwhile, Regen has signed up to play Charlie and Louis's assistant Ro-Ro, while Davis has joined the cast as Renata, the bookkeeper for Charlie's fiancé Ali.
Regen has appeared in shows such as Whoopi, The Black Donnellys, Sherri and Entourage, while Davis's television credits include Studio 60 on the Sunset Strip and the original British The Office.
Ugly Betty star Michael Urie has already landed the role of Louis, while One Tree Hill's Sophia Bush will play Ali in the pilot.