“Directing is 80/90 percent casting” has been credited to directors Elia Kazan (1909-2003), William Wyler (1902-1981), John Ford (1894-1973), John Huston (1906-1987), Martin Ritt (1914-1990) and Robert Altman (1926-2006). The saying means that casting is most important to the success of a production (either a play or a film).
   
“When casting’s done, 90 percent of my creative work is done” was said by film director Robert Altman in 1977. “[Elia] Kazan used to say to us at Lincoln Center, it’s 80 percent casting, 10 percent skill, and 10 percent luck” was cited in print in 1986. Kazan possibly said this at least as early as 1961, when he directed Warren Beatty in Splendor in the Grass. It’s not certain if William Wyler, John Ford or John Huston ever said it.
   
   
4 September 1977, Boston (MA)

, “Altman’s special ‘Wedding’ breaks film-making pattern” by Ruth Hamilton, pg. B5, col. 6:
“When casting’s done, 90 percent of my creative work is done. The roughly 48 people we’ve got here, how the hell can they go wrong? They’reall terrific.”
(Robert Altman—ed.)
 
Google News Archive
30 September 1978, The Real Paper, “Perfect Couple,” pg. 18, col. 1:   
He (Robert Altman—ed.) repeats his well-known analysis that “casting is 90 percent of the creative work in filmmaking,” noting that he tends simply to “put people into situations and just let the rest happen.”
 
Google News Archive
23 July 1981, The Day (New London, CT), “Meryl Streep in title film role of William Styron’s ‘Sophie’s Choice’” by Aljean Harmetz (N. Y. Times News Service), pg. 20, col. 1:
“Casting is 80 percent,” Pakula says.
(Film director Alan J. Pakula, 1928-1998—ed.)
 
Google Books
Behind the Filmmaking Scene
By Virginia Phelps Clemens
Philadelphia, PA: Westminster Press
1982
Pg. 40:
Robert Altman, who directed Popeye, feels that casting is 90 percent of the creative work.
 
Google Books
Off Camera:
Conversations with the Makers of Prime-Time Television
By Richard Levinson and William Link
New York, NY: New American Library
1986
Pg. 94:
Gary Goldberg: That was my most important job, because I’ve become convinced that the success of a television series is ninety percent casting.
   
Google Books
Theatre Times:
A Publication of the Alliance of Resident Theatres/New York
Volumes 6-9
1986
Pg. 108:
ML (Mindy Levine): I’ve heard many directors say, casting is 90 percent of directing a show. Do you believe that?
HS (Harold Scott): It is—80 percent anyway. [Elia] Kazan used to say to us at Lincoln Center, it’s 80 percent casting, 10 percent skill, and 10 percent luck.
 
10 June 1990, Miami (FL) Herald, “Hill climbs to pinnacle of action directing,” Amusements, pg. 11:
If directing is 90 percent casting, as William Wyler once said, then Hill is 90 percent there.
 
Google Books
The Films of Martin Ritt:
Fanfare for the Common Man
By Gabriel Miller
Jackson, MS: University Press of Mississippi
2000
Pg. II:
Marty said directing was 80 percent casting.
 
New York (NY) Times
THEATER; Onstage, Ambivalence and Despair
By ALVIN KLEIN
Published: November 16, 2003
(...)
If, as it is often said,’’ directing is 90 percent casting,’’ you can do the math when it comes to tallying Paul Carlin’s score. But 10 percent remains, and that’s hard to figure.
   
Google Books
Warren Beatty:
A Private Man
By Suzanne Finstad
New York, NY: Three Rivers Press
2005
Pg. 201:
Kazan, who believed that directing was 80 percent casting, and was hen a great admirer of Freud, analyzed the characters in his scripts and chose actors whose personalities mirrored them, based on his psychoanalysis of the actors.
 
Google Books
Extraordinary Board Leadership:
The Keys to High Impact Governing
By Doug Eadie
Sudbury, MA: Jones and Bartlett Publishers
2009
Pg. ?:
The story is told of a film student who asked the great director John Houston if he was aware that 50 percent of the success of his films had to do with casting. Houston is said to have replied, “Dear boy, my films are 100 percent casting!”
     
Google Books
The Enraged Accompanist’s Guide to the Perfect Audition
By Andrew Gerle
Milwaukee, WI: Applause Theatre & Cinema Books
2011
Pg. ?:
There’s an old saying that “directing is X percent casting,” with that number being anywhere from 75 to 90. The vast majority of American theater productions have between two and four weeks to rehearse, tech, and open a show. This is not enough time to teach someone how to act, generate empathy, get a laugh. or exude that special quality a particular role requires.
   
Backstage
VIDEO: Top NY Casting Directors on ‘Working in the Theatre’
By Daniel Lehman | Posted Feb. 13, 2012, 5:04 p.m.
(...)
In an hour-long conversation, moderator Patrick Pacheco speaks with New York casting directors David Caparelliotis, Tara Rubin, and Daniel Swee about the casting director’s job, their collaboration with directors, recasting shows, and whether stars and “names” are necessary for success on Broadway.
 
“An axiom in show business maintains that directing is 90 percent casting,” Pacheco says.
 
Atlantic City (NJ) Weekly
Ben Affleck’s ‘Argo’
Oscar buzz for a based-on-true-events drama
By Lori Hoffman
Posted Oct. 10, 2012
(...)
On his cast: If I was lucky with casting my previous two movies, this was [winning] the lottery. John Ford said directing is ninety percent casting and that is in evidence here. What John [Goodman] and Alan [Arkin] were able to do, taking the Hollywood satire element of the movie and keeping it realistic while making it funny was incredibly challenging but necessary in order to stitch the movie together.