Interesting question Tony! I've
never really thought about it, but my gut feeling says that
speech is interrupted when there's no space between the last
letter and the m-dash; if there's a space, it may lead the reader
to think that though the sentence is incomplete, the speaker
intended to break off where he/she did, and intends to finish the
sentence (and will have the opportunity to do so, say, after a
speech tag).
Here are the rules in
brief, from my nifty little "Elements of Grammar":
1) Use a dash to indicate an abrupt change in a sentence.
"We see words that blow
like leaves in the winds of autumn
– golden words, bronze words, words that catch the light
like opals." (JJ Kilpatrick, The Writer's Art)
2) Sometimes a dash is used to set
off interpolated explanatory matter.
There are many
differences
– aside from the physical ones
– between men and women.
3) Use a dash to indicate a sudden
break in a sentence. (When a sentence ends in a dash, no period
is needed.)
"Let me know if you ever
need–"
She broke off in tears.
4) A dash may be used to set off a
long phrase in apposition, particularly when the phrase is
punctuated with commas.
"The male rampant –
killing animals for food and clothing, digging out caves, and
putting up huts, driving off enemies –early came to be associated
in the mind of the elemental female with warmth, well-being,
safety, and the kindred creature comforts." (James Thurber,
"Listen to This, Dear")
5) A pair of dashes may be used
instead of parentheses.
“All branches of the family
produced their individual eccentrics – there was even an uncle
who believed in the Single Tax – but they were united in their
solid understanding of the value of money...” (K.A. Porter,
“Gertrude Stein: A Self Portrait”)
6) Use a dash instead of the word
‘to’ in reference
to dates, pages, paragraphs, verses, and cantos.
1910 – 1940
Pages 10 – 49
Paragraphs 1 – 14
Genesis 2: 10 – 14
Verses 5 – 10
Cantos I – IV
I hope that helps!