“The foundation of acting is the reality of doing” is a saying that has been printed on many images.  The saying is credited to acting teacher Sanford Meisner (1905-1997).
   
“The foundation of acting is the reality of doing” was printed in the book Sanford Meisner on Acting (1987) by Sanford Meisner and ‎Dennis Longwell.
     
     
Wikipedia: Sanford Meisner
Sanford Meisner (August 31, 1905 – February 2, 1997) was an American actor and acting teacher who developed an approach to acting instruction that is now known as the Meisner technique. While Meisner was exposed to method acting at the Group Theatre, his approach differed markedly in that he completely abandoned the use of affective memory, a distinct characteristic of method acting. Meisner maintained an emphasis on “the reality of doing”, which was the foundation of his approach.
 
The Meisner technique
Meisner’s unusual techniques were considered both unorthodox and effective. Actor Dennis Longwell wrote of sitting in on one of Meisner’s classes one day, when Meisner brought two students forward for an acting exercise. They were given a single line of dialogue, told to turn away, and instructed not to do or say anything until something happened to make them say the words (one of the fundamental principles of the Meisner technique). The first student’s line came when Meisner approached him from behind and gave him a strong pinch on the back, inspiring him to jump away and yelp his line in pain. The other student’s line came when Meisner reached around and slipped his hand into her blouse. Her line came out as a giggle as she moved away from his touch.
   
Google Books         
Sanford Meisner on Acting
By Sanford Meisner and ‎Dennis Longwell
New York, NY: Vintage Books
1987
Pg. 16:
“The foundation of acting is the reality of doing.”
It is the first moment of the first class of the semester, and without delay Sanford Meisner states and restates this seemingly simple theme. “Wait a minute, let’s say that again. The foundation of acting is the reality of doing. The reality of doing. Now, how do you know what that means? I’ll clarify it.”
 
31 May 1987, New York (NY) Times, “Acting Class, Day One, With a Master Teacher,” pg. H5, col. 2:
Sanford Meisner’s credo: ‘The foundation of acting is the reality of doing.’
(Excerpt from Meisner’s book, cited above.—ed.)
     
Google Books
The Angst of American Acting:
An Assessment of Acting Texts
By Cecilia Jessica Pang
University of California, Berkeley, thesis dissertation
1991
Pg. 285:
“The foundation of acting is the reality of doing” says Sanford Meisner.
   
Google Books
David Mamet:
A Life in the Theatre
By Ira Nadel
New York, NY: Palgrave Macmillan
2008
Pg. 9:
As Mamet’s influential drama teacher Sanford Meisner, founder of the Neighborhood Theatre Playhouse in New York, would repeat, “the foundation of acting is the reality of doing” (SMA 16).
 
Twitter
Laura Merryweather
@laMERRYWEATHER
“The foundation of acting is the reality of doing” Meisner - He know’s his stuff!
1:56 PM · Nov 22, 2009
     
Google Books
Double Shakespeares:
Emotional-Realist Acting and Contemporary Performance
By Cary M. Mazer
Lanham, MD: Rowman & Littlefield
2015
Sanford Meisner, who routinely began his acting classes with the axiomatic declaration, “The foundation of acting is the reality of doing,” used what he called the “word repetition game,” in which the actor begins by responding to the immediate reality of sensation and the exercise-partner’s words, to help the actor create a foundation of genuine response to genuine stimulus—to shorten the distance between what Meisner called “the pinch and the ouch” that could, later in the actor’s training, be expanded to include the imagined circumstance of the role.
 
Twitter
Acting Magazine
@ActingMagazine
“The foundation of acting is the reality of doing.”
(Sanford Meisner)
11:30 PM · Oct 31, 2022