If I were asked to divide up the roles appropriate to authors of plays/novels and the film makers who make their words into movies, I would say that the film makers should make their best attempt at framing in their specific, selected view of the original work of literature. That is to say, they are not to change the story or dialogue too severely but concentrate on creating a reality for viewers to see, hear, and perceive out of the written work.
A film I think achieves the goal of creating a carefully tuned reality is A Streetcar Named Desire, directed in 1951 by Elia Kazan, based on Tennessee Williams' 1947 play. Kazan selected a cast natural to their roles, and he directed each scene so carefully that lighting alone can tell viewers enough about the inner struggles within certain characters.
Kazan stuck to Williams' original dialogue very tightly. The main artistic shift he made was the significant attention to the character. Stanley Kowalski. Played by Marlon Brando at the peak of his physical beauty, Stanley completely takes over the first few scenes in which he appears. Brando makes it clear that Stanley knows how to control any situation, even if he does it with something as shallow as his muscles. I was pleased that Stanley is portrayed this way. Stanley cannot have possibly been as remarkable in written form because the power of his presence, heavily shaped by good looks, is hugely visual.
Kazan also makes an ingenious move by casting the withering Vivien Leigh as Blanche DuBois, who is also withering. Vivien Leigh's personal reality is very similar to Blanche's fictional one, and she is able to provide viewers with another stellar performance.
In addition to Streetcar, I find Anthony Asquith and Leslie Howard's 1938 cinematic version of Pygmalion, a 1913 adaptation of George Bernard Shaw's play, to be highly worthwhile. This film also keeps the original dialogue intact. Again, casting is very successful. Leslie Howard plays the major role, Professor Higgins. He fleshes Higgins out nicely--he is not too aged or harsh-faced, and he has an indescribable zeal to his movements that makes his sharp attitude something one can watch.
Another character to admire in the flesh is Eliza's father, Mr. Doolittle. The literary version of the character is rich and well-developed in its own right, but the sound of Wilfrid Lawson's corrupt chuckle is as worthy as his spiel about middle-class morality.
Wendy Hiller, who portrays Eliza, is also topnotch, and one way we can rate her is by comparing her to Audrey Hepburn as Eliza in George Cukor's 1964 film, My Fair Lady. Hepburn is a fine actress, but her perfectly formed eyes, nose, and cheekbones give her away as one of the upper class. She looks more privileged because she cannot look bad in any amount of dirt. Hiller works better because she is the type who cleans up well. Viewers can see a direct change when she goes from flower girl to duchess.
My Fair Lady is an example of a less worthwhile adaptation. It is framed in a romantic plot and focuses on a love affair that is nonexistent in Shaw's play. Although a lot of the dialogue still remains, it comes off as less natural. The cast brings very little to the characters' believability. The main characters, Eliza and Higgins (Rex Harrison), do not exude a mutual chemistry. Elaborate costumes and musical skits add some merit to the viewing experience; but, overall, I did not find this adaptation to be very consistent with the play.
Having an opportunity to compare and contrast the different adaptations all at once has really helped me decide what makes a film good. Ultimately, the film maker has to make a lot of careful decisions and pay attention to the way their choices work to form a sincere imitation of life.