I always said I'd get round to doing one of these for James one day. I've tried to stick reasonably close to CQ letter canon, which may explain some important omissions. :)
1. James grew up with his elderly mother and father in Canterbury. From there, it wasn't too far to the seaside, which is where he spent most of his days and evenings during the holidays. When he was older, and Sirius was staying, they used to go further afield, but he always had a great fondness for the seaside.
2. His first girlfriend was a Muggle girl called Lizzie, when he was fourteen and she was almost sixteen. He was terribly proud of having lost his virginity to an older woman, even though the actual relationship only lasted all of a week and a half - ending when James stood her up in favour of exploring the ruins of the pier in the harbour with some of the older boys.
Despite this, they parted on good terms and remained in contact for the rest of James's life. Lizzie was one of the few people to believe James died in a car accident as he was never quite able to tell her the truth about himself.
3. James never admitted he was proud to be a pureblood. He had nothing against Muggles, but was much happier being nothing but a wizard and not a Muggle - as Muggles couldn't do magic and thus led much more boring lives and were to be pitied. However, as his parents were fiercely dedicated to being blood-traitors, James often found himself on the receiving end of criticism regarding their beliefs and quickly learnt to be every bit as vehement and outspoken as was necessary to put people off picking a fight with him.
But the fact remained that James was proud to be a pureblooded wizard, feeling very much that the magic was a part of him as it was in his blood - which made his blood more exciting. On becoming friends with Sirius, and seeing all the hardships Sirius was forced to endure because of his refusal to endorse blood purity, James felt it would be a betrayal to say anything.
4. At about the age of thirteen, James decided he was going to marry Lily Evans. He decided it in the middle of a Potions lesson, while Sirius was diverting Slughorn and James was sneaking under the tables to nick some billywig stings from the Potions cupboard. He passed by her legs, paused, decided he fancied her and, unsure how else to proceed due to the strength of his feelings, hexed her socks (the hex being a mild one to turn her socks to jelly).
5. James had something of a fixation on the pureblood families, most notably the Blacks. He kept up-to-date with pureblood society, as his parents expected of him, but always took a special interest in the activities of the Blacks. Naturally he never mentioned this to Sirius, but he studied the Blacks with the distinct air of a scientist studying another form of life.
To James's mind, Sirius elevated himself out of the Blacks and was not to be counted as one of them. But James kept a close eye on how the family managed without Sirius and was very disappointed to see Narcissa marry Lucius and spent many hours discussing with Sirius what exactly the duties of the heir to Black House were.
6. On the morning of his wedding, James decided that it was all a ruse of Lily's to pay him back for being such a bastard at school. He was convinced that she was planning to publicly humiliate him, and was only persuaded to go to the proceedings when Sirius promised to be there right by his side. James had never been able to tell Sirius no.
7. On learning that Harry could possibly be the child referred to in the Prophecy, James cast every defensive and protective charm on Harry that he knew - including several very old, very Dark pieces of magic. None of them made the slightest bit of difference and it was only Lily's sacrifice that saved Harry, but that was one of only two times that James used Dark magic.
8. Since the age of four, James had the same prophetic dream of Godric's Hollow nine separate times. In the dream, the house was destroyed and he could hear a baby crying. Each time, he felt the imperative need to find the baby, but was unable to. He didn’t understand what the dream meant until he was standing face-to-face with Voldemort.
9. Due to their advanced age, James tried very hard to keep his parents from hearing all of his escapades at Hogwarts. This did not lead to him behaving better however, merely becoming more proficient in covering his tracks.
His parents died within six months of each other in the first year of James leaving school. He was absolutely devastated and it was losing them both that prompted him to propose to Lily.
10. James was exceptionally similar to his uncle William. It was a badge of pride at first, until he discovered how badly William had torn his family apart. After that, James despised the Dark Arts in all forms. The one time his father and he had a serious falling out was due to James's decision to join the Order. His father told him he was still too much like William, and James never really got over the upset.
12. The Prank was when James realised just how much he'd give up for Sirius. On the way to intercept Snape, he was already planning where to hide the body if he were too late.
13. Before he actually mastered the Animagus transformation, James desperately hoped his animal form would be a bird of some kind - preferably a kestrel. The discovery that he was a big prey animal did not please him at all. However, his antlers did become a source of great pride to him and he was thrilled to read, during his research, that his antlers were above average size (having already had Sirius measure them for him).
That said, while the antlers were growing the first time, James suffered from terrible headaches and migraines.
14. James felt very much the father of the Marauders. While he loved and respected them all, he considered them to be in his care. Remus was the only one whom he believed could probably look after himself. Sirius was his beautiful brother whom he adored, and was therefore to be protected at all costs, and nobody but the Marauders were smart enough to give Peter the credit he was due so it was up to James to make sure nobody gave him any trouble.
James was aware of many of his flaws, but never that he could be immensely patronising.
15. Callum was James' oldest friend. He was five years older and was the son of some of the Potters' friends. The two boys had little in common besides Quidditch, but nonetheless kept in contact and told each other everything - well, almost everything, James never betrayed Remus's secret or revealed that they had mastered the Animagus transformation.
When the war became particularly nasty, Callum retreated into the Muggle world with his Muggle wife and two children. He and James lost contact at that point, agreeing to meet up when things were safer. Callum didn't return to the wizarding world until 1983, and only learnt of James's death then. He has since followed Harry's presence in the papers closely.
16. James had a natural talent for flying and, coupled with his outstanding reflexes which also stood in him very good stead for duelling, it was not surprising that he was such an exceptional Quidditch player. The Montrose Magpies were not the only club to have noted his performance and to be considering making him an offer, the Magpies simply got there first.
17. Two things in the world could be counted on to bring James out of a mood: Sirius Black and flying. After he got together with Lily, the list extended to Lily and eventually Harry, who, unbeknownst to Lily, he sometimes took flying on his broom with him.
18. The other time James used Dark Magic was the night after he learnt of Regulus's death. Moody argued his case to Crouch, explaining that James was under stress due to the war. James was sent home to Lily and Harry, but he never discussed the circumstances.
19. When Voldemort arrived at the door, it took James exactly twenty-three seconds to realise his mistake. The remaining time was spent begging Lily and Harry's forgiveness, and praying for Sirius and Remus.
20. James knew he couldn't beat Voldemort.