

It is entirely possible that James simply learned about the spell just like everyone else. But this proposition doesn’t quite hold its ground as we have had Remus also mention how this spell had become quite popular in their fifth year, which is incidentally also the time when we see James use it on Snape. The theorists also go on to make a connection between the spells written in Half-Blood Prince’s potions book. They point out that James is the only other person we witness using the Levicorpus spell written in the potions book claiming James, not Snape, invented the spell. In any case, it’s quite rare to maintain a consistent handwriting style for over a decade. Snape’s described to be scribbling away, but Harry fails to register this detail. We did have a chance to get a glimpse of Snape’s teenage handwriting in Chapter 28 of Harry Potter and the Order of the Phoenix, where Harry manages to have a look at Snape’s worst memory. This theory is backed by the fact that not just Harry but even Hermione failed to recognize the handwriting of the same man who had been their potions master for five years. The second theory suggests that the particular textbook originally belonged to James, but Snape stole it from him and wrote “This book is the property of The Half-Blood Prince” on it just to be petty. This suggests that perhaps James Potter, while an overall brilliant student, wasn’t particularly gifted in potions. However, we have had Slughorn mention Snape and Lily as some of his favorite students, with no particular mention of James in this regard.

On the other hand, this theory is also backed by Snape’s need to beat James as well as to impress Lily Evans, who was Slughorn’s star student. We know Remus is no good in potion-making, and Peter is described by McGonagall as a less than average student, leaving only James and Sirius to the task.

The idea of James Potter being at least slightly talented potions is entirely possible, especially since potion brewing is a part of the process of becoming Animagi that requires tremendous skill. From whatever little we know about James, we won’t put it past him to be that annoying kid that tries to show off every chance he gets. The first one proposes that Snape could’ve scribbled down these hacks while eavesdropping on James’s conversations with his friends during the Potions classes.

With that in mind, there are two ways in which James could’ve influenced the Half-Blood Prince’s findings. It’s also been mentioned a couple of times that James was one of the top students in his class, evidencing his cleverness. Even without being particularly gifted, James would’ve been familiar with some tips and tricks of the trade by simply seeing his father work. He invented the infamous Sleekeazy Potion, which quadrupled the family gold with its massive success in Britain as well as all the way across the Atlantic in the United States. Fleamont Potter, James’s father, was an accomplished potioneer. This theory was born out of a deeper insight into James Potter’s family and their careers. James Potter is the real brain behind the clever tips and tricks written in the Half-Blood Prince‘s textbook and not Severus Snape.
