In Animal Man comics written by Grant Morrison, the evil and crazy Psycho-Pirate has become aware of the comic-book-reader audience and is trying to goad his army of resurrected super-villains into attacking them. (Morrison's entire run is about the growing awareness of the characters that they're in a comic book.) The peak of this is probably when the hero has a mind-expanding peyote trip, looks out of the frame at the reader and cries "OH MY GOD! I CAN SEE YOU!" When Animal Man meets Morrison, he flies into a rage and kills him, shocked at his actions, Buddy freaks out, only to see Morrison standing behind him. Morrison tells Buddy that he can't be killed, and those actions and rage Buddy felt, Morrison wrote. Morrison says that he's not there, it's only an Author Avatar, and he can't really interact with Buddy, implying that Psycho Pirate could never leave the comic and get into the real world, all of their actions are driven by the author, even when they think they aren't.
DC used this on occasion, under the claim that "Earth Prime" was the reader's home dimension, and so any threat to the multiverse was a threat to the reader. This... stopped working. Hey, remember how the universe was destroyed by a wave of antimatter in 1985, and suddenly reappeared in 2006? Me neither.
A Fridge Logic-y version occasionally happens with The Joker. He never outright states he can see you or interact with you, but he does interact with his own speech balloons and has turned the page for the reader, indicating that he knows you're out there. Now, consider what the Joker tends to do to people...
At one point in the Joker's Asylum: Two Face special, Joker tells the audience to get a coin and flip it to decide how the story ends. Mr. J is threatening enough towards the reader that most people who read it actually physically flipped a coin and read the ending it indicated.
The character Superboy Prime is supposed to come from Earth Prime, our world, and has even posted on the real-life DC message board. The trope comes in effect when you remember that Superboy Prime is a Psychopathic Manchild with no compulsions against killing everything and everyone, with the same powers of Superman only much stronger. You can start screaming in horror.
Alan Moore wasn't safe from one of his creations. John Constantine from Hellblazer visited him... and talked to him... in real life... not once... but TWICE! It scared the hell out of him.
A lot of the captions in issue #1 are threatening warnings along the lines of "You think this is just a comic, but it's bait. You're bait for THEM." In his human identity Nix Uotan is reading the same comic, including scenes from his own life, and that's how he's pulled into the pan-dimensional crisis.
Thunderer claims that The Gentry are "pitiless ones from behind the Invisible Rainbow". Given how he uses rainbows to refer to other universes, there's the implication that the fourth wall is incapable of protecting the comic book characters from The Gentry.
Several pages in Pax Americana #1 seem designed to resemble an eye. The implication is... not promising.
Ultra Comics #1, the Real-World Episode of the series, has its hero warning potential readers that their universe will be endangered if this magazine is read. At the end, you get infected by The Gentry.
In issue #2, the newly formed Operation Justice Incarnate trace the Gentry's origin to Earth-33, and state they're coming after them. Oh, Crap!...
The penultimate issue of Zero Hour: Crisis in Time! and all tie-in issues released that week had their few last pages as blank white pages as the entropy rifts hits in the stories.
In Infinite Crisis the villain, Alexander Luthor, is smashing realities together in order to create the best one. He notices the reader, our reality, and reaches out for it.
