If a character starts a sentence that he intends for another to finish (but they don't), does the sentence still end with an em dash?
If the speaker is interrupted, use an m-dash; if s/he just dribbles off, use an ellipsis. Is that what you are inquiring about? Ah, on re-reading, I see what you're after. Both punctuations create moods: the m-dash would represent a sudden stop and rapt anticipation; the ellipsis would produce a sense of hopelessness of a response from the second speaker. That's how I feel about it.
either way, the readers won't be up on the fine line of distinction between the two, so just be consistent... if you're lucky enough to get a publisher, the editor and house rules will have the final say, anyway...
