I Know This Much Is True

Mark Ruffalo

Dominick Birdsey / Thomas Birdsey

Melissa Leo

Concettina Ipolita Tempesta 'Ma' Birdsey

Rosie O'Donnell

Lisa Sheffer

Archie Panjabi

Dr. Patel

Kathryn Hahn

Dessa Constantine

John Procaccino

Ray Birdsey

Rob Huebel

Leo

Michael Greyeyes

Ralph Drinkwater

Gabe Fazio

Shawn Tudesco / Dominick and Thomas Acting Double

Juliette Lewis

Nedra Frank

Philip Ettinger

Early Adulthood Dominick Birdsey / Thomas Birdsey

Aisling Franciosi

Young Dessa

Marcello Fonte

Domenico Onofrio Tempesta

Imogen Poots

Joy Hanks

Rocco Masihi

Young Thomas Birdsey

Donnie Masihi

Young Dominick Birdsey

Laura Esterman

Ruth Hood

Bruce Greenwood

Dr. Hume

Joseph Ragno

Henrey Hood / Henrey Rood

Annie Fitzgerald

Miss Hanker

Acahkos Johnson

Penny Ann Drinkwater

Jaiden Klein

Marie Sexton

Charles Reina

Channy

Violet Adams

Bunny Borsa

Oliver Patnode

Otero

Matt Helm

Young Leo

Derek Goh

17-19 Year Old Dominick & Thomas Acting Double

Agatha Nowicki

Angie Constantine

Irene Muscara

Prosperina Tucci

Adrienne R. Williams

Mrs. Birdsall

Tatanka Means

Nabby Drinkwater

Harris Yulin

Father Lavie

Sue Jean Kim

Dr. Yup

Michael Simon Hall

Stuart Feinstein

Gina Costigan

Mrs. O'Meara

Steven Cox

Boyfriend of Dr. Hume's Daughter

MorningStar Angeline

Nabby Drinkwater's Wife

Al Linea

Abruzzi

Richard Fitch

Mill Foreman

Amy Lynn Stewart

Merciful Mother

Guillermo Díaz

Sergeant Mercado

Marylouise Burke

Librarian

Brian Goodman

Al

Tim Ruddy

Robocop

Sandra Berrios

Social Worker at Hatch

Laura Silverman

Kristin

Christine Parks

Humanities Department Chair

Morgan Sullivan

Office Buddy

Jake Robards

Landlubbers (voice)

Michael Genet

Thomas' Surgeon

María Gonzalez

Ma's Oncology Nurse

A.C. Peterson

Gene Diogenes

Christopher J. Hanke

Thad

James Quattrochi

Bus Driver

Rachel B. Joyce

Jeanette

Paul Coleman

Support Group Counselor

Brandon J. Dirden

Kyle's Dad

Katt Masterson

Doreen

Jason Furlani

Larry Penn

Kate Grimes

English Professor

Chelsea Sheets

Brunette

Felix Solis

Nurse

Joe Grifasi

Steve Falice

Raymond Neil Hernandez

Omar

Simone Coppo

Vincenzo Tempesta

Sean Cullen

Dr. Yates

Federico Dordei

Nunzio Tucci

Shannon Plumb

Charlotte

Zaria Degenhardt

11 Year Old Concettina

Mark A. Gamma

Mourner Mark

Wayne J. Miller

Mourner Wayne

Thomas Gionet-Lavigne

Ray's Orderly

Alexander Hacker

Templeton

Robert Mladinich

Paddy Wagon Officer

Destry Allyn Spielberg

Dr. Hume's Daughter

Roberta Riganò

Ignazia Tempesta