- Doctors and medical students
- Stanley Tucci as Dr. Kevin Moretti, Chief of Emergency Medicine (season finale only)
- Amy Aquino as Dr. Janet Coburn, Chief of Obstetrics
- John Aylward as Dr. Donald Anspaugh, Chief of Staff
- Leland Orser as Dr. Lucien Dubenko, Chief of Surgery
- J. P. Manoux as Dr. Dustin Crenshaw, Surgical Chief Resident
- Gina Ravera as Dr. Bettina DeJesus, Radiologist
- Maury Sterling as Dr. Nelson, Psychiatrist
- Sara Gilbert as Dr. Jane Figler, Second Year Resident
- Busy Philipps as Dr. Hope Bobeck, Intern
- Julia Ling as Mae Lee Park, Medical Student
- Kim Strauss as Dr. Ari, Anesthesiologist
- Malaya Rivera Drew as Katey Alvaro, Medical Student
- Marc Jablon as Dr. Larry Weston, Intern
- Perry Anzilotti as Dr. Ed, Anesthesiologist
- Ethan Hova as Dr. Ken Maser
- Aasif Mandvi as Dr. Manish
- L. Scott Caldwell as Dr. Megan Rabb, Director of Neo-Natology
- Dahlia Salem as Dr. Jessica Albright, Surgical Chief Resident (outgoing)
- Nurses
- Deezer D as Nurse Malik McGrath
- Laura Cerón as Nurse Chuny Marquez
- Yvette Freeman as Nurse Haleh Adams
- Lily Mariye as Nurse Lily Jarvik
- Dinah Lenney as Nurse Shirley
- Angel Laketa Moore as Nurse Dawn Archer
- Kip Pardue as Nurse Ben Parker
- Kyle Richards as Nurse Dori Kerns
- Nasim Pedrad as Nurse Suri
- Tane Kawasaki as Nurse Claire
- Mary Heiss as Nurse Mary
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- Staff, Paramedics and Officers
- Abraham Benrubi as Desk Clerk Jerry Markovic
- Troy Evans as Desk Clerk Frank Martin
- Glenn Plummer as Desk Clerk Timmy Rawlins
- Tara Karsian as Social Worker Liz Dade
- Charlayne Woodard as Angela Gilliam (from Staff Services)
- Emily Wagner as Paramedic Doris Pickman
- Montae Russell as Paramedic Dwight Zadro
- Lyn Alicia Henderson as Paramedic Pamela Olbes
- Michelle C. Bonilla as Paramedic Christine Harms
- Demetrius Navarro as Paramedic Morales
- Brian Lester as Paramedic Brian Dumar
- Louie Liberti as Paramedic Bardelli
- Brendan Patrick Connor as Paramedic Reidy
- Vyto Ruginis as Flight Paramedic Wright
- Christopher Amitrano as Officer Hollis
- Joe Manganiello as Officer Litchman
- Chad McKnight as Officer Wilson
- Bobby Nish as Officer Danny Yau
- Louis Iacoviello as Officer Rovner
- Family
- Sally Field as Maggie Wyczenski
- Fred Ward as Eddie Wyczenski
- Sam Jones III as Chaz Pratt, later Paramedic
- Dominic Janes as Alex Taggart
- Garret Dillahunt as Steve Curtis
- Lois Smith as Gracie (Sam's grandmother)
- Paula Malcomson as Meg Riley
- Chloe Greenfield as Sarah Riley
- George Gerdes as Jim Riley
- Deka Beaudine as Helen Riley
- Stacy Keach as Mike Gates
- Rosalee Mayeux as Jacy Barnett
- Andrew Gonzales and Aidan Gonzales as Joe Kovač
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