I recently wrote an article declaiming the use of the exclamation point in fiction, but mollified my fervor with the reluctant admission that there were indeed exceptions. And, in some instances, even an occasional benefit if the mark is used judiciously. But the employment of the parenthetical expression in fiction is not afforded the same luxury. And for three very good reasons:
The Action of a Parentheses is often the Opposite of its Intended Function
The most problematic issue concerns the use of a parenthetical expression for emphasis, when the punctuation is designed as a means to express a derivative meaning or "aside." In the instance of a writer wanting to accentuate the narrative, a dash or dashes should be utilized. Someone once suggested a simple check and balance for what to select in which circumstance: Consider a parentheses like two walls muting the text in between, while a dash, as in adding a dash of spice to a meal, heightens the flavor of the textual bill of fare. Perhaps a hokey explanation, but one I never forgot.
There is another Issue with Parentheses in Fiction that goes much Deeper
Once more, the evil Showing versus Telling monster exposes its fangs, and long ones this time. This is because a parenthetical expression inherently tells of an action that could've and often should've been shown. Simply, if the writer deemed the "aside" important enough to set off with specialty punctuation, wouldn't what fostered the exposition be worth detailing substantively for the reader? Ask again the critical question, was this rhetoric within the parentheses provided for modest purpose, such as clarity, or was it positioned within the punctuation to enhance the narrative? If the rationale is the latter, there is a distinct possibly that a valuable Showing opportunity in the crafting of a plot point was missed.
Then there is Patronizing the Reader
A large number of readers find nothing more grating than having situations or things explained to them by via parenthetical supplements. If the reader can't figure out the narrative because it is so weak that it requires reinforcement, this is often an indication the novel needs a serious rewrite, with a focus on Showing the scene or scenes which are being cloaked, and not Telling them. Many experts feel that parentheses have no place in the narrative of a work of fiction, and it might be wise for writers who wish to be published to heed this opinion.