Ronald Weasley is a fan-favorite character, as are most Harry Potter characters. Still, Ron has one of the more relatable personalities as a misfit, well-meaning, loyal brat.
Rupert Grint play a part of more than just Ron, though, as the actor makes an appearance in the books. Harry Potter and the Half-Blood Prince, released after the fourth movie (Goblet of Fire), has references to Grint upon having met Rowling beforehand.
Professor Slughorn was a pompous character that cared only for himself and the elite students he personally deemed worthy. If you weren't spectacular, as Ron's normal personality and lower-middle-class legacy would presume, you weren't very memorable in his eyes. Thus, when Slughorn mentions Ron in the 2005 Bloomsbury edition of Harry Potter and the Half-Blood Prince on page 454, he makes an oddly familiar verbal slip.
"Rupert," obviously in reference to Ron, is meant to not only belittle Harry's friend by accident, but also to reference the actor himself.
An interesting, thoughtful, and subtle reference, you wouldn't expect Rowling to casually mention the actor, yet here it is, mentioned amongst plenty of other words and paragraphs.
Hidden in plain sight, a significant amount of foreshadowing is done through dialogue in Rowling's books. The only question is whether or not there are other references to actors within the books.
Now armed with this information, go back and hunt for references. We know Potterheads are going to do it anyway, whether they're given an incentive or not!