We never really learn a lot about what happened between James Potter and Severus Snape in the Harry Potter movies. But the books do give us some insight and it does make further speculation easier.
Looking at the movies, we only know what Dumbledore told Harry – that James saved Snape’s life. But – the how, why, when, and where – we can only speculate. The answer to this question lies in the past, at the time when the Marauders attended Hogwarts, which is from 1971 to 1978.
Why were the Marauders against Snape?
Snape was already attracted to dark magic before he got his letter of invitation to Hogwarts. He already knew more dark spells in his first year than graduates. Potter and the gang, who were all Gryffindors, did not approve of this.
But more to the point, James and Snape were love rivals. They both loved Lily Evans. However, Snape was her childhood friend, and Lily possibly already loved him when they came to Hogwarts. However, be it by Snape’s or James’ actions, or just by the mysterious acts of fate, Lily fell in love with James.
Snape Hated James, and Vice Versa
Because Potter systematically seemed to undermine Snape and Lily’s relationship every chance he could get, Snape developed a deep hatred for the man. They had numerous duels, many of which almost had serious and fatal consequences.
When did James Potter save Snape’s life?
It was during one of Remus Lupin’s transformations while they were all at Hogwarts. Since all the other members of the Marauders learned to transform into animals to keep Lupin company, they became suspicious of Snape.
Sirius Black wanted to play a trick on Snape for undisclosed reasons and told him the way to disable the Whomping Willow. Black possibly meant this as a practical joke or he really wanted Snape to get hurt.
But, when Snape followed Remus one night on the full moon, he would have been in mortal danger, had it not been for James Potter. Black told James of the practical joke he played on Snape and James did not like it.
He was against the man, but this seemed like too much. He arrived there just in time to pull Snape back, but not before Snape caught a glimpse of Remus in his werewolf form. Dumbledore forbids him to tell anyone. The entire thing is explained in the third book by Lupin.
An excerpt from the book
“Severus was very interested in where I went every month,” Lupin told Harry, Ron, and Hermione. “We were in the same year, you know, and we — er — didn’t like each other very much. He especially disliked James. Jealous, I think, of James’s talent on the Quidditch field… anyway Snape had seen me crossing the grounds with Madam Pomfrey one evening as she led me toward the Whomping Willow to transform. Sirius thought it would be — er — amusing, to tell Snape all he had to do was prod the knot on the tree trunk with a long stick, and he’d be able to get in after me. Well, of course, Snape tried it — if he’d got as far as this house, he’d have met a fully grown werewolf — but your father, who’d heard what Sirius had done, went after Snape and pulled him back, at great risk to his life… Snape glimpsed me, though, at the end of the tunnel. He was forbidden by Dumbledore to tell anybody, but from that time on he knew what I was…”
Taken from the book Harry Potter and the Prisoner of Azkaban, by J. K. Rowling
Whether James felt bad for Snape, or he didn’t want Lily to hate him forever is unknown. But he did save him for sure, for even the most powerful wizards are not safe against the savagery of a werewolf.
James saved Snape’s life that night, and Snape never forgave him for it. Snape felt like he took Lily from him, but he drove her away on his own, more likely with his dark tendencies and short temper. However, these events did not end as most other love quarrels do.
Snape became a Death Eater because of his broken heart
Severus Snape was a more powerful wizard than all the Marauders combined. But there was nothing he could do to bring Lily back, once he lost her. It is at that moment of weakness that Voldemort approached him. He knew about the situation, as Peter Pettigrew presumably told him, or by other means.
Voldemort seduced Snape to his cause and Snape did betray Lily and James to him when he told Voldemort a part of the prophecy connecting him and Harry. It is for this reason that he went to kill the Potters, and Snape never forgave himself for that.
Here is a brilliant and award-winning fan-made video made by Broad Strokes. It involves those crucial moments when tensions between Snape and the Marauders escalated. And, when Voldemort appeared like a balm on the wound.