In act one when Yuri says she has been reading horror books they say they didn't expect that from a person who's so gentle .In act two Monika says she expects that from a person like her. I noticed that and thought it would be interesting. There is also a chance when your writing poems in act two that Monika's hidden sticker will jump in the air.
Viewing post in Easter eggs and hidden details in this game
I actually played the whole game through to check that out. It’s sooo cool but kinda weird too!! It goes to show how much deeper this game goes than we thought. And I thought the sticker that might jump up was a Sayori sticker in act 2, but I just checked it, and it’s soo much creepier that it’s Monika!!