One of things I've noticed over the years is that the most memorable characters are usually memorable based on what they do within the campaign.
I have a concept of what they are like before play, but it never survives the first sessions or two perfectly intact.
For me, the process of discovering who a character is rather than preconceiving character in overly detailed fashion is pretty important. And that is virtually impossible without exposing that PC to the pressure cooker of gameplay wherein actions taken when stakes matter often determine who that PC is. Often that can be part of mechanical build, like when my wife's (@Nephis ) Wizard in our Dungeon World game used her Storm Aura spell (a custom move that allows the Wizard to discharge Hold either to cast a lightning bolt or ride the wind a la Storm from X-Men) to leap down a well alone to confront the source of a miasma animating the skeletal remains of a dragon graveyard against us. This character from before Session Zero was framed as impulsive, but this particular instance defines for me (and for the rest of the group, I think) what that impulsive nature really means and looks like.Hmm. Character actions can certainly be memorable, but when I think back over recent campaigns, those aren't usually the elements that stand out the most. I recall a heated argument between two PCs about whether or not they should torture a prisoner. Or a scene where one of the PCs was betrayed by a trusted NPC. Or when a character retired from the campaign after his two brothers were killed. Game mechanics and character abilities weren't involved in any of these scenes. Their gravitas was generated by the backstories of the characters involved.
This is certainly true for me, too. I usually start with a reasonably detailed sense of who I think the character is. This gets filtered through my own roleplaying capabilities (not terrible, but I'm not an actor). Some characterizations are easier for me to accomplish than others. (And some require too much spotlight time, so they get deprecated.) My conception of a character also changes significantly after I interact with the other PCs.
When I'm GMing a game where some personality traits are mechanized (like Disadvantages in GURPS), I explicitly allow players to retroactively adjust those traits over the first few sessions as they get to know their characters.
This might seem like disagreement, but I'm not sure it is. I'm curious what you think.I think PC actions, regardless of mechanical in/outputs, fundamentally determine PC character. And those actions require at least a modicum of actual gaming episodes to manifest.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.