So you have a film scrip or an action plot for an ad that involves a character or characters that are a bit left-of-center or simply are considered eccentric. In that case, using a quirky voice is something you'll have to look for.
There are many ways to define an eccentric character, but when you say that somebody is quirky, you are certainly giving your description a positive slant. It can describe somebody's general demeanor, or just his sense of humor. But, whenever you want to represent such a character, or the message or text contains elements of quirkiness, you would need an adequately quirky voice to represent it to make it believable.
Any audience can relate to a certain does of eccentricity, and particularly quirky humor. Feature film audiences are usually the main targets of a quirky voice, but then, any audience will react positivelly to a certain dose of a quirky humour, it can be a movie trailer, a new book or music album that contain a certain does of humor that will make its audiences react positively to a quirky voice. This is particularly true of teenage and young adult audiences, which always positively react to quirky humor.
A quirky voice can vary both in its tone and pace. It can go from very high to very low, from ver fast-paced, to extremely slow. A typical voice that is considered quirky is that of actor Pee-wee Herman, who used a variable pitch and pace in his films and TV series.