I don't get why Bill's fear was... (spoilers)
His little brother, who he was searching for. If it is about facing your fears, how is that a fear? He was facing his wishes.
shareHe was afraid he was dead, that's why he was in denial about it.
shareWhen he encountered Georgie in the sewer and Bill put a bolt into his head, I actually thought for a few seconds that he may have just killed him there and then. Of course, I realise that would be silly as his arm was torn off and he would've bled out by then, plus he'd already seen him in the basement of his house as he rotted away in front of him.
There are four major hints/giveaways/red flags/etc. that it was IT/Pennywise as Georgie. 1) IT/Pennywise appears as Georgie in the miniseries when they are in the sewers. 2) Same thing with the novel (I think). 3) Georgie showed up earlier in the movie (basement scene). 4) Georgie led Bill directly to where Beverly was, which was IT's lair. And IT/Pennywise was trying to separate all the kids. IT/Pennywise failed though on killing Stan since the others stopped him, so IT/Pennywise changed into Georgie to try to get Bill to wander off from the group. Also, I'm pretty sure that Georgie appeared where IT/Pennywise disappeared to after the kids stopped him killing Stan.
A fifth clue/hint would probably just be if you are able to figure out plots of movies or anything with a story/plot (books, games, movies, etc.) For me it was mainly this reason, but everything else quickly followed or came to me two seconds later.