Well, Joker (um, the forum member) seemed surprised that Joker was in this movie, but given this is the spoiler thread, I guess we can be more specific.
The clip can probably be found on youtube, but really, I'm not sure what more I can say other than it appears to be the complete Riddler/Joker scene, and plays out exactly as the reports have been saying. I find Barry's voice a tad high pitched, almost as if he were attempting to do a Heath Ledger impression. He appears to be seen mostly in profile behind the glass of his cell.
The shot of Riddler kicking the wall of his cell comes from this scene. I can't describe what Joker looks like really because it's so dark. I can tell you that he has spiky hair. My first reaction was that it's the kind of spiky hair you'd see in a 90's boy band, that kind of wet look.
He also has a high pitched cackle, although I didn't really get the hyenah sound others had mentioned. But the scene concludes with the two laughing, as the camera zooms out of the hallways of the prison.
I'd describe his laugh as very childish sounding, oddly enough. Someone said he sounded like Ledger. I don't hear anything of Ledger in his voice. It just sounds like Keoghan's regular voice tbh.
