What is the proper way to phrase such a question, with respect to the em dash placement?
Did the grocery store have the milk on sale--I thought they did, but I'm uncertain?
Is it best to separate these, like such:
Did the grocery store have the milk on sale? I thought they did, but I'm uncertain.
This is where your writer's hat comes on. Listen to the subtle differences in tone.
The first one keeps an inquisitive tone throughout the sentence, but the dash gives a hurried or pushed feeling, like the speaker leaves only a soft pause at most between the question and the explanation.
The second one gives more of a concrete separation and leaves it sounding more natural.
On the other hand, I might even do a variation of these, such as:
Did the grocery store have the milk on sale--thought they did, though I'm not certain.
The thing I like about this is it's rhetorical and declarative and it builds character. It makes the character more self centered, more interested in hearing himself speak than the answer itself. The dash is almost like an indicator he isn't willing to wait for the character he's talking to to respond.
There are, I'm sure, countless variations. A tweak here or a tweak there could change how readers view the character.
Of course I could be full of it but that's my uneducated opinion, if you want it
