Search found 2 matches: David Lean

I've got 2, before the averages drop off.
Ernest Lehman -
[spoiler]Who's Afraid of Virginia Woolf? (1966)
Sweet Smell of Success (1957)
North by Northwest (1959)
Sabrina (1954)[/spoiler]
Ben Hecht
[spoiler]The Shop Around the Corner (1940)
Rope (1948)
Wuthering Heights (1939) (adapted)
His Girl Friday (1940)
Where the Sidewalk Ends (1950)
Notorious (1946)
Scarface (1983)[/spoiler]
Otherwise:
Thomas Harris (author)(Hannibal Lecter)
Bibhutibhushan Bandyopadhyay (author)[Ray films]
Noël Coward/Ronald Neame(also a director and all-round ledge, apparently) [David Lean]
Krzysztof Piesiewicz [Kieslowski]
Julius J. Epstein

2016 Update


I'm forced to exclude great writers like Scola, Frank Pierson, David Leland, and Campanile, who became full-time directors at some point in their careers.

#. Writer................. Avg (films ranked)

1. Ennio Flaiano......... 81.80 (10) ...mostly for Fellini.
2. Suso Cecchi d'Amico. 79.36 (14) ...for Visconti, Monicelli, Antonioni, De Sica and Rosi.
3. Tullio Pinelli.......... 79.14 (14) ...mostly for Fellini.
4. Rodolfo Sonego....... 78.60 (5) ...for Risi, Zampa, Monicelli, De Sica and Comencini.
5. Marshall Brickman.... 76.20 (5) ...wrote early Woody Allens.
6. Bernardino Zapponi.. 75.67 (6) ...for Fellini, Monicelli, Comencini, Argento...
7. Jean Aurenche........ 75.57 (7) ...for Clément, Autant-Lara, Tavernier and Carné.
8. Agenore Incrocci..... 75.18 (17) ...often with Scarpelli, for Monicelli, Scola, Risi and others.
9. Lorenzo Semple Jr.... 74.20 (5) ...for "Papillon", and a bunch of great thrillers.
10. Terence Rattigan..... 74.00 (6) ...playwright, mostly adapted by Anthony Asquith.
11. Gyula Hernádi....... 73.83 (6) ...mostly for Miklós Jancsó.
12. Cesare Zavattini..... 72.67 (6) ...mostly De Sica plus Visconti's "Bellissima".
13. Jacques Prévert...... 72.60 (5) ...mostly Carné.
14. Furio Scarpelli........ 72.53 (15) ...often with Incrocci, for Monicelli, Scola, Risi and others.
15. Graham Chapman.... 72.14 (7) ...Monty Python.
16. Ruggero Maccari..... 71.92 (12) ...for Pietrangeli, Risi, Scola and others.
=17. Irving Ravetch....... 70.80 (5) ...mostly Martin Ritt.
=17. Samson Raphaelson. 70.80 (5) ...for Ernst Lubitsch.
19. Mario Puzo............ 70.20 (5) ...Godfather, Superman.
20. Michael Wilson....... 69.60 (5) ... for Lean, Schaffner, Capra and George Stevens.

Honorable mentions: Eric Idle (82.75/4), Monica Mcgowan Johnson (76.5/4), Arlette Langmann (75.25/4), Jean-Claude Grumberg (74/4), Calder Willingham (72.5/4), Brunello Rondi (71.5/4)

Mardik Martin was very close to making it on my list, when I saw "The Cut". Harriet Frank Jr. was foiled by "Silver River".