According to the after-notes of Of Mice and Men, Steinbeck had decided that the novel should be considered a 'playable novel'. This means that, without any additions, subtractions, etc., a theater company could used the novel as its play book. The descriptions were the setting and stage direction, the dialogue was for the actors, and so on.
When you look closely at Of Mice and Men, his attempt is supported because there is no description of emotion that isn't done through an action of some kind. Steinbeck doesn't say that Lenny feels hopeful when he talks about the rabbits. He doesn't say that George feels bogged down by Lenny except through dialogue and action.
While his attempt was supported and you can see the underpinnings once you know what he was attempting, the plays and screenplays that followed the book did have to be written separately. (It's really hard to memorize lines among all that narration.) However, the book itself is wonderful because of the drama-like tension that's created in the scenes. The story stretches taut and practically thrums because there's no one filling you in to anticipate what will happen. You have to 'watch' the action.
Then I got to thinking of all the genre-mixing and what it could lead to. What if he'd decided to write 'playable-poetry'? How many genres are there to mix up and create something new? Epistolary novels are even a genre mix--letters and novels. I've heard of poetic novels (Elizabeth Barrett Browning did one way back).
I don't have any clear inspirations yet, but it didn't make my mind wander a bit. What genres--and by genres here I mean types of writing (plays, poetry, screenplays, novels, etc.) not subject matter--do you think would be interesting or effective to mix up? What effects do you think it would have?