Example:
"Exploding whales?" My date stared at me as if I had given him some bad news. He'd taken me to this fancy Italian restaurant. "That's what your thesis is about?"
I'm not sure why, but this sounds better to me than a. Can I use this in this way in fiction?
I think in this case this serves to emphasize the object in the sentence and connotes a specific kind of consideration or attitude (the narrators?) towards the object . On a natural level I find it quite reasonable - but I don't have a precedent or formal reference to support that.
You have a first person narrator, you're trying to convey a sense of character, and this is commonly used in this way by (some) people in informal speech, so it's perfectly okay and appropriate to use in this situation.
Most of the rules for formal and academic writing can be blithely disregarded when you're writing dialog or a first-person narrative, because you're trying for a reasonable approximation of casual conversation, and in real life people break all the rules.
It changes the meaning for me.
The use of "this" implies to me that you are still at the restaurant in question, whereas the use of "a" would imply that you were no longer there. "a" also implies that you don't care which restaurant it was, only that it was fancy.
As stated the use of this in place of a is a Southern colloquialism. It would be okay to use if the story is first person and the protagonist/narrator is native to the appropriate region.
It should be noted: it immediately gives the sense of the narrator being a impoverished uneducated Southerner. It pushes more emphasis on her opinion of how fancy the restaurant is, and implies she unused to such places.
If the story was written in third person, the grammar would be expected to conform with traditional grammatical standards since it's an semi-omniscient narrator telling the story.
No. It obfuscates the meaning of your sentence and impedes thought flow; especially in the case given, which I would rewrite completely for clarity.
[ironic] Edit for Clarity: I say that it confuses and makes understanding your sentence more difficult because I couldn't even tell how I was supposed to read the sentence at first. I actually thought it was a fragment. I had to re-read it a few times before I figured out what you were trying to say.
I say that it impedes thought flow because it breaks me out of your story; it totally distracted me from the interesting, relevant, first sentence about the 'thesis'. Granted, it would have been less distracting if I had immediately understood the sentence, but when I say that I would rewrite it for clarity I mean that I personally would probably reorganize the setting information out of the middle of the dialog.
But why this restaurant?