I just read an article about how the Phoenix Suns basketball team destroyed the LA Lakers in last night's game. (Grinning because I love the Suns and hate the Lakers.) The following sentence is in the article:
Saturday comes the real test for Phoenix: Golden State on Christmas Day.
When reading that sentence, I missed the colon so I had to stop when I read the incomplete sentence that followed it and then went back and saw the colon for it to make sense. If the colon had been an em-dash, I wouldn't have missed it. And I believe it would have been more impactful.
True, the article isn't a fiction story, but I believe it carries over to fiction.
Maybe it's just my love of the em-dash. Or maybe I can't resist the opportunity to mention the Suns beat the Lakers. LOL