Everything is in the scarf.
Despite a few plot gaps, the Harry Potter series did a fantastic job of paying attention to detail. This holds true for both literature and films. Many elements were communicated in subtle visual ways in the films. Here’s an excellent illustration.
We see Harry, Ron, and Hermione wearing scarves at this moment. What’s more, guess what? Their individual characteristics are reflected in the way they wear their scarves. Hermione’s is neat and tidy, Ron’s is sloppy and disorganised, and Harry’s is somewhere in the middle.