What is the radical difference between ‘this’ and ‘a’ when telling a story?

Question

The following quotation is a line from Ron to Harry after the first stage of the Triwizard Tournament. (p359, Harry Potter 4, US edition)

“You were the best, you know, no competition. Cedric did this weird thing where he Transfigured a rock on the ground … turned it into a dog … he was trying to make the dragon go for the dog instead of him. Well, it was a pretty cool bit of Transfiguration, and it sort of worked, because he did get the egg, but he got burned as well – the dragon changed its mind halfway through and decided it would rather have him than the Labrador; he only just got away. And that Fleur girl tried this sort of charm, I think she was trying to put it into a trance – well, that kind of worked too, it went all sleepy, but then it snored, and this great jet of flame shot out, and her skirt caught fire – she put it out with a bit of water out of her wand. (The rest is omitted.)

I consulted dictionaries.

・(informal) used when you are telling a story or telling somebody about something.

eg) There was this strange man sitting next to me on the plane.

eg) I’ve been getting these pains in my chest.

It seems to me that I can use ‘this’ for a change whenever I’m bored with ‘a’.

・(Informal) an emphatic form of a or the: used esp on relating a story

eg) I saw this big brown bear

I can’t understand what it emphasizes. What does ‘an emphatic form of a’ mean?

I’ve seen this type of ‘this’ in some stories, but they are used only once in opening scenes, if I’m not mistaken. I tried to swallow ‘this’ by thinking it is needed for an opening scene which is very important in the story. But I can’t delude myself any more when seeing three ‘this’ in the above quotation. What kind of effect repeating ‘this’ have?

Let me get it straight. I’d like to know

1. What makes you choose ‘this’ over ‘a’?
2. What kind of effect does repeating ‘this’ have as in the above citation?

Answer 1

There is a big difference in connotation between a and this with regards to storytelling. It wouldn't do to simply replace all of the occurrences of one with the other; unfortunately, the subtleties are hard to pin down.

So there was this guy...

This focuses a stronger attachment to this particular guy. The story isn't about any ol' guy; the guy itself is a critical piece of the story.

So there was a guy...

A reduces emphasis on the particulars of the guy. The particular guy in the story isn't that interesting or important; the important stuff comes afterward.

Cedric did this weird thing...

The weird thing is, again, critical. Specifically, the weirdness of the thing is probably what switches the usages from a to this. Cedric doing this thing means that it was that particular thing that caused the events.

Cedric did a weird thing...

At this point, attention is typically shifting off of the weird thing. The events after the weird thing are what really matter.

The carryover from other uses of this may make this easier to see:

I want this car

I want a car

Another way to think of it is to replace this with this particular:

Cedric did this particular weird thing...

And that Fleur girl tried this particular sort of charm...

All of that being said, the everyday use of this usage tends to overplay, and therefore defeat, its purpose:

So this guys does this thing where he wants into this bar and gets this drink that has this color and he drinks it this way.

By signaling that nearly everything is important, nothing actually is. I actually consider the last use in your example to fall prey to this overuse:

... and this great jet of flame shot out...

There isn't actually anything interesting about the great jet of flame; the interesting part of that sentence is the skirt catching fire. The more you use it, the less meaningful it becomes. Use it too much and you probably delve into Buffy Speak.

Answer 2

I think that the word emphatic was poorly chosen. The intention is to give your story a sense of immediacy; suddenly it's not about people or objects in the abstract, but (nearly) concrete individuals, e.g. not just

"Cedric did a weird thing (that I'm going to describe to you)"

but

"Cedric did this weird thing (that you are witnessing as well at the same time as me because I am relating the story as though it is happening right now)."

(thanks to @Kit.)

Of course, it doesn't really work; the story doesn't really become any more real to your listener, and if you over-indulge in this technique the constant repetition of "this" starts to be a bit annoying. For this reason, most people (of my US English-speaking acquaintance and reading, at least) stop talking this way after high school; writing dialogue that uses this technique is a good way to establish your characters' age. Definitely "informal."

Answer 3

Use 'this' when you're referring to something specific:

• 'I use this kind of potion' [to point toward a particular variety of potion]

Use 'a' when you are speaking more ambiguously:

• 'I use a kind of potion' [to speak of potions more broadly]

In other words, if you're speaking of a set, use 'a'; if you're speaking about a particular element of that set, use 'this'.

Answer 4

This seems like an attempt at informal speech. It isn't meant to emphasize anything, except perhaps the way normal people speak.

And the characters are normal people (within the context of a Harry Potter story).

Answer 5

This only happens in informal speech, so there are no written down rules of grammar for this usage. I can tell you what I think, though, but I don't have any references to back me up and I may be completely wrong, so feel free to downvote me. My impression is first that this is typically used at the start of stories. Here,

Cedric did this weird thing ... And that Fleur girl tried this sort of charm ...

are both the starts of the sections of the story about Cedric and about Fleur. The this here is used to indicate that a new topic is being introduced.

Second, it's used for emphasis; it emphasizes the thing it modifies. So here it indicates that "this great jet of flame" was something impressive.

You can, of course, overuse the technique by replacing nearly every a with this.