Demonstratives usually help us point at things we consider identifiable and, in some sense, already known by our communicative partners. In Spanish, whenever I say something like este gato ‘this cat’, it is either because the cat is present in the physical situation ‒ besides, it is implied that the cat is comparably close to me, since I would probably have said ese gato ‘that cat’ or aquel gato ‘that cat over there’ if it were not ‒ or because I have previously mentioned it. In the latter case, I see the reference to the cat as retrievable from the context by those listening or reading ‒ even if probably not as easily retrievable as if I had said él ‘he, it’ or just indexed it through e.g. a verbal agreement morpheme, such as third-person verbal ending -ó in subió a un árbol ‘climbed up a tree’, or a possessive, as in sus ojos brillaban ‘its eyes were gleaming’. In typically variegated linguistic jargon, we might say that the cat is relatively ‘salient’, ‘accessible’ or ‘activated’ in the context.
This has always made some uses of English demonstratives quite puzzling to me, as in the following ‒ more or less real ‒ example, where I would hardly use anything other than a where the speaker uses this:
So I was walking down the street today and I ran into this guy that was ranting about the end of the world…
In the first line of this (haha) song by Ed Sheeran we directly come across this previously unknown girl. The usage is repeated to exhaustion in a Bob Sponge clip, even if the joke does not seem to be on the demonstrative itself but on linguistic recursiveness. From informal conversation with a number of English native speakers I’ve gathered that they do not see the phenomenon as remarkable in any sense. And yet the possibility of using demonstratives with previously unknown and contextually unrecoverable referents appears to be rather unusual crosslinguistically. It is still true that the use of ‘presentational this‘ would not be equally natural in all contexts; the proposed example is one where the referent is not necessarily alien to the conceptual frame. In other words, it is not unlikely in itself to run into a guy on the street, however weird his behavior may be ‒ would it be possible to say I ran into this martian with four eyes that was talking in a verbless language? Provided you’re not a generativist, that is.
English demonstratives are undoubtedly quite versatile as regards their semantic and pragmatic possibilities. The uses alluded to here pose particularly interesting questions regarding the differences between definite and indefinite gramatical units ‒ definite articles being known to come from demonstratives in different languages, Romance ones obviously among them ‒ as well as the configuration of noun phrases at large and its relationship with discourse and cognition.