I'm not a native English speaker and sometimes I encounter article "the" used in stressed form before some noun or name, mostly in movies, like "That's not the gun, that's THE gun" or "He is THE specialist". Is it correct or common use of this article for emphasizing something outstanding? Is it allowed in written form?
Having 'THE' emphasized draws attention to the article, stressing the fact that this is 'the' gun, as opposed to 'a' gun... in other words, it is highlighting that it is a specific gun, not simply one gun out of many.
It is generally used to point out that something is in some way exceptional or highly relevant.