I Know This Much Is True (TV) Cast
The cast of I Know This Much Is True. Updated on October 2, 2025.
78 actors
Mark Ruffalo
as Dominick Birdsey / Thomas Birdsey (6 episodes)

Mark Ruffalo plays the dual roles of Dominick Birdsey and Thomas Birdsey in I Know This Much Is True. Dominick is a complex character dealing with his own personal demons while also caring for his mentally ill brother, Thomas. Thomas, who suffers from schizophrenia, presents a unique set of challenges for Dominick as he tries to protect and support his brother. Ruffalo's performance showcases his versatility and ability to portray the emotional depth of both characters.

Melissa Leo
as Concettina Ipolita Tempesta 'Ma' Birdsey (6 episodes)

Melissa Leo portrays Concettina Ipolita Tempesta 'Ma' Birdsey in I Know This Much Is True. Ma Birdsey is the matriarch of the Birdsey family and a source of strength for her sons, Dominick and Thomas. Leo's performance captures the resilience and love that Ma Birdsey exhibits as she navigates the difficulties faced by her family. Melissa Leo's portrayal brings depth and authenticity to the character of Ma.

Rosie O'Donnell
as Lisa Sheffer (6 episodes)

Rosie O'Donnell takes on the role of Lisa Sheffer in I Know This Much Is True. Lisa is a social worker who becomes involved in Dominick and Thomas' lives as she tries to help them navigate their challenges. O'Donnell's performance as Lisa showcases her ability to bring both warmth and determination to the character, providing support and guidance to the Birdsey brothers.

Archie Panjabi
as Dr. Patel (6 episodes)

Archie Panjabi portrays Dr. Patel in I Know This Much Is True. Dr. Patel is a psychiatrist who plays a significant role in the lives of Dominick and Thomas Birdsey. Panjabi's performance brings intelligence and compassion to the character, as she works with the brothers to understand and manage their mental health issues. Archie Panjabi's portrayal adds depth and empathy to Dr. Patel.

Kathryn Hahn
as Dessa Constantine (6 episodes)

Kathryn Hahn takes on the role of Dessa Constantine in I Know This Much Is True. Dessa is Dominick's ex-wife and the mother of his child. Hahn's performance captures the complex dynamics of their relationship and the challenges they face while co-parenting. Kathryn Hahn's portrayal adds depth and complexity to the character of Dessa.

John Procaccino
as Ray Birdsey (6 episodes)

John Procaccino portrays Ray Birdsey in I Know This Much Is True. Ray is Dominick and Thomas' stepfather and a supportive figure in their lives. Procaccino's performance captures Ray's empathy and understanding as he tries to guide the Birdsey brothers through their struggles. John Procaccino brings authenticity and compassion to the character of Ray.

Rob Huebel
as Leo (6 episodes)

Rob Huebel plays the character of Leo in I Know This Much Is True. Leo is a close friend of Dominick's and a source of humor and support in his life. Huebel's performance brings levity and warmth to the character, providing a balance to the emotional intensity of the series.

Michael Greyeyes
as Ralph Drinkwater (6 episodes)

Michael Greyeyes portrays Ralph Drinkwater in I Know This Much Is True. Ralph is a childhood friend of Dominick and Thomas and plays a significant role as their paths intertwine. Greyeyes' performance showcases the complexity of Ralph's character, adding depth and intrigue to the series.

Gabe Fazio
as Shawn Tudesco / Dominick and Thomas Acting Double (6 episodes)
Harris Yulin
as Father Lavie (1 episode)
Bruce Greenwood
as Dr. Hume (2 episodes)
Marcello Fonte
as Domenico Onofrio Tempesta (2 episodes)
Sue Jean Kim
as Dr. Yup (1 episode)
Michael Simon Hall
as Stuart Feinstein (1 episode)
Gina Costigan
as Mrs. O'Meara (1 episode)
Steven Cox
as Boyfriend of Dr. Hume's Daughter (1 episode)
MorningStar Angeline
as Nabby Drinkwater's Wife (1 episode)
Al Linea
as Abruzzi (1 episode)
Richard Fitch
as Mill Foreman (1 episode)
Amy Lynn Stewart
as Merciful Mother (1 episode)
Guillermo Díaz
as Sergeant Mercado (1 episode)
Marylouise Burke
as Librarian (1 episode)
Brian Goodman
as Al (1 episode)
Tim Ruddy
as Robocop (1 episode)
Sandra Berrios
as Social Worker at Hatch (1 episode)
Laura Silverman
as Kristin (1 episode)
Christine Parks
as Humanities Department Chair (1 episode)
Morgan Sullivan
as Office Buddy (1 episode)
Jake Robards
as Landlubbers (voice) (1 episode)
Michael Genet
as Thomas' Surgeon (1 episode)
María Gonzalez
as Ma's Oncology Nurse (1 episode)
A.C. Peterson
as Gene Diogenes (1 episode)
Christopher J. Hanke
as Thad (1 episode)
James Quattrochi
as Bus Driver (1 episode)
Rachel B. Joyce
as Jeanette (1 episode)
Paul Coleman
as Support Group Counselor (1 episode)
Brandon J. Dirden
as Kyle's Dad (1 episode)
Katt Masterson
as Doreen (1 episode)
Jason Furlani
as Larry Penn (1 episode)
Annie Fitzgerald
as Miss Hanker (2 episodes)
Kate Grimes
as English Professor (1 episode)
Joseph Ragno
as Henrey Rood (1 episode)
Acahkos Johnson
as Penny Ann Drinkwater (2 episodes)
Jaiden Klein
as Marie Sexton (2 episodes)
Charlie Reina
as Channy (2 episodes)
Violet Adams
as Bunny Borsa (2 episodes)
Oliver Patnode
as Otero (2 episodes)
Chelsea Sheets
as Brunette (1 episode)
Matt Helm
as Young Leo (2 episodes)
Felix Solis
as Nurse (1 episode)
Laura Esterman
as Ruth Hood (3 episodes)
Joe Grifasi
as Steve Falice (1 episode)
Derek Goh
as 17-19 Year Old Dominick & Thomas Acting Double (2 episodes)
Raymond Neil Hernandez
as Omar (1 episode)
Simone Coppo
as Vincenzo Tempesta (1 episode)
Sean Cullen
as Dr. Yates (1 episode)
Federico Dordei
as Nunzio Tucci (1 episode)
Agatha Nowicki
as Angie Constantine (2 episodes)
Rocco Masihi
as Young Thomas Birdsey (4 episodes)
Irene Muscara
as Prosperina Tucci (2 episodes)
Adrienne R. Williams
as Mrs. Birdsall (2 episodes)
Shannon Plumb
as Charlotte (1 episode)
Tatanka Means
as Nabby Drinkwater (2 episodes)
Donnie Masihi
as Young Dominick Birdsey (4 episodes)
Zaria Degenhardt
as 11 Year Old Concettina (1 episode)
Mark A. Gamma
as Mourner Mark (1 episode)
Wayne J. Miller
as Mourner Wayne (1 episode)
Thomas Gionet-Lavigne
as Ray's Orderly (1 episode)
Alexander Hacker
as Templeton (1 episode)
Robert Mladinich
as Paddy Wagon Officer (1 episode)
Philip Ettinger
as Early Adulthood Dominick Birdsey / Thomas Birdsey (2 episodes)
Aisling Franciosi
as Young Dessa Constantine (2 episodes)
Destry Allyn Spielberg
as Dr. Hume's Daughter (1 episode)
Imogen Poots
as Joy Hanks (3 episodes)
Roberta Riganò
as Ignazia Tempesta (1 episode)
Juliette Lewis
as Nedra Frank (2 episodes)
Tim Ruddy
as Robocop (1 episode)
Ethan Crystal
as Thin Sailor (1 episode)
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