I'm just about to play this VN, do you guys have any suggestions what I should pay attention to?
I mean, is there a route for each character (like most other VNs) that needs to be played first?
The story and characters are more independent from each other. For example if Dave (you) is meant to react to somthing in the story that will happen no matter who's route you are on, the routes are what flavour you want to go along with the story. Who will be at daves side when he reacts.
When you can look at the name of the coffee cup just put 'Dave'. The cup isn't for changing your name it unlocks some easter eggs but otherwise it just gives you +1 affection with whatever character's name is on it. Putting 'Dave' gives +1 to everyone which can help if you want to get some of the better interactions like hugging someone when they're sad.
Passwords are usually in the bad ending and is a word that is usually repeated or has more attention drawn to it. If it isn't in the bad ending it's usually in the same day you can go to the vault. The early days (day 1-3) you can go to the vault and thats just for easter eggs not for a needed password.
The choice that locks in what route you are on is when you pick who your partner is for the scavenger hunt.