If the lights went out and the movie screen went dark, and only the characters' voices were left, would the audience still be able to understand the story? Look at Shakespeare. (Yesterday I said I wouldn't look at Shakespeare, but I lied.)
Duke. If music be the food of love, play on; Give me excess of it, that, surfeiting, The appetite may sicken and so die.
Read any play by Shakespeare. Even without his sparse stage directions, his stories are perfectly clear. Passion, despair, laughter-- it all comes across in the dialogue.
Let's look at a novelist: Margaret Mitchell. She wrote brilliant dialogue in Gone With the Wind-- so that when the screenplay was written, most of the lines were lifted directly from the book.
"Isn't it enough that you've collected every other man's heart here today?...Well,you've always had my heart, you know. You cut your teeth on it."
Finally, let's look at the sort of dialogue that the author of How to Write a Movie in 21 Days holds up as an example:
INT. KITCHEN - MORNING
SUE cooks breakfast. MAX enters. Caresses Sue.
They sink to the floor in the folds of matching thick robes.
I will take Shakespeare any day over this. Turn off the lights and the movie screen, and "Mmmm" means almost nothing.
Juliet. Spread thy close curtain, love-performing night, That runaways' eyes may wink, and Romeo Leap to these arms...Come, night; Come, Romeo; come, thou day in night... Give me my Romeo, and, when he shall die, Take him and cut him out in little stars, And he will make the face of heaven so fine That all the world will be in love with night And pay no worship to the garish sun.
-Romeo and Juliet