Funny, we hear and read a lot of discussion about "novel vs. film" and the faithfulness of the latter to the fomer, but not so much "play vs. film," unless it's Shakespeare or some other stage classic. Lately, though, I've been curious about some of the plays my favorite films are based on.
I just finished reading Susan Sandler's Crossing Delancey, for instance, a play about a young Jewish woman living and working on the Upper West Side of Manhattan, who is infatuated with a self-absorbed writer while being gently courted by a Lower East Side pickleman. I love the 1988 film adaptation starring Amy Irving and Peter Riegert, and although the play differs somewhat (there is no David Hyde Pierce character in the play, for instance), I quite enjoyed it, almost as much for the differences as for the play itself. The biggest thing I would miss if I saw a production of the play is New York -- the film makes the city a character in itself, and you get a real sense of the uptown/downtown lifestyle conflict in the story. (Of course, now you have to watch the film, if you haven't already, in order to know what I'm talking about!) However, I imagine the stage version, which calls for three small sets on the stage at once and has the main character moving from one set to another and back again, would have a more personal, immediate feel, and the character's struggle between her downtown roots and her uptown ideals would be reinforced by the proximity of the different sets. At any rate, both film and play are sweet romantic comedies and worth the time to watch/read.
Some months ago, I read Philip Barry's Holiday. The 1938 film version starring Katharine Hepburn and Cary Grant is one of my favorite of both actors' films: Hepburn is wistful and strong-minded in a role that seems tailor-made for her, and Grant exudes down-to-earth charm. There is an earlier version (1930, with Ann Harding -- whose performance earned her an Oscar nomination -- and Robert Ames), but it isn't available for commercial purchase. The 1938 screenplay by Donald Ogden Stewart stays quite close to Barry's stage script, so if you only see the movie and never read the play, you wouldn't really be missing out. Much of the film feels very "stagey" anyway, especially the scenes in the playroom. (No, the Kate Winslet/Cameron Diaz film, The Holiday, is not based on the Barry play!)
At the moment, I'm reading The Time of the Cuckoo, by Arthur Laurents (pronounced "Lawrence"). The lovely 1955 David Lean film, again starring Kate Hepburn, this time teamed with Rossano Brazzi, is another great favorite of mine. As in the Crossing Delancey movie, the location (in this case, Venice) is itself a character, and Jack Hildyard's cinematography is sigh-invoking. I see on the film's IMDb page that the author H. E. Bates co-wrote the screenplay with Lean, but that Donald Ogden Stewart had an uncredited hand in it. It's a wonderful screenplay, of course, but the stage script almost reads like a novel -- Laurents, in his stage directions, writes illuminatingly about his characters so as to make them even more vivid for both actors and casual readers. In fact, one almost wishes Laurents had written a novel instead of a play; I think the story would make a wonderful novel, something perhaps Elizabeth Bowen could well have done. If you like the film, reading the play would be very much worth your time.