I noticed by doing promotions you'll get the idol to come back at night only. I think the same goes for lessons too. Just a small trivia.
I also noticed during stage for you, that you can't toggle the dynamic camera as you could in One for All. Like, in One for All, during certain parts of the song you can press right to toggle between views. But, it doesn't seem like you could do this in Platinum Stars, so it's kind of like Idolm@ster 2 in that aspect. Of course, in Platinum Stars you have another camera mode where the camera moves towards the idol at different angles.