La Première Sortie – Pierre-Auguste Renoir
The advantage which a novelist has over a dramatist is that he is always there to explain his characters. He can occupy twenty pages in analysis of his heroine’s thoughts as she wonders whether to wear the pink or the heliotrope. On the stage you merely see her in pink. If the hero is to commit a murder, his soul will have been laid bare before you in a couple of chapters. On the stage, now that soliloquy is out of date, the hero can only tell a convenient friend about it; and even then it will be pointed out to the dramatist that one doesn’t usually discuss one’s projected murders with a friend. In a book, the curate’s sense of humour and the politician’s sentimentality are their own, not the author’s; but, listening to them in a play, you may tell yourself that the author is not very funny and very much too sentimental.
For this reason, since so much is left to its intelligence, a dramatist relies upon his audience; hopefully, if not always with confidence.
– Michael and Mary by A.A. Milne
The Captive Reader 
I love it when he discusses these things! Have you read The Great Broxopp yet? He discusses that brilliantly in his autobiography – how to make a genius on the stage, when his words have to be the playwright’s words.
I haven’t read The Great Broxopp yet – have to save something for 2013!