Live roleplay games can vary greatly in format and setup, so a few words before I begin to explain my background and what experience I'm drawing on. I've run twice weekly sessions for our local university game for many years, but my favourite format remains our annual weekend-long event with prewritten characters. Players fill in a form about the sort of character they'd like to play and then we spend about ten months writing them detailed character briefs that are intricately interwoven with both the world setting and each other. There tend to be around 50-60 player characters. I've been heavily involved in writing ten of these games by now and I have learnt quite a lot along the way.
1) The FIVE THINGS format is good quick and dirty way to conceptualised and write up a culture.
Live roleplay games, especially those in the United Kingdom, tend to be set in fantastical worlds and thus the need to write good, clear culture briefs is immense, especially as players are prone to bringing with them assumptions and extrapolations. And I want more from my fictional cultures than just palette-swapping the familiar. The five things format asks questions of how a culture sees and thinks of itself. As the format is also very focused on how a character relates to the setting material and how it can be in roleplay, it can be very useful when it comes to creating characters for the page.
The format is simple. Three setions and in each is listed: a) five things everyone in this culture knows; b) five ways to insult someone from this culture; c) five ways to flatter someone from this culture.
Five things that are known asks me to really drill down into what is important to someone in this culture. They can be thought of as simply facts that they know and hold dear (eg: "Everyone knows our city is in the centre of the world, both metaphorically and literally.") or a rather more open ended prompt (eg: "Everyone has a favourite patron saint whose story they draw inspiration from").
I've mentioned it before in my blogpost Five Things about Writing Fictional Cultures and there is a little bit more detail there about the format.
2) It's sometimes okay to embrace the cliches.
I've always been a writer intensely averse to cliche, to point that I find myself paralysed when all obvious resolutions have pages on tvtropes. Writing backstories and plots for players who want to play the Big Damn Heroes and the Dastardly Villains. Remixing old tropes is half the fun for the players, to be themselves a version of a beloved character.
So we have worlds of secret identities and backroom deals and dark promises. I learnt to write indulgently, leaning into tropes and touchstones. Because I writing for an audience who wanted the familiar so that they can put their own mark upon it, it was no bad thing for them. Furthermore, each plot tangled together the traditional roles, and the dashing knight of one plot would harbour a dark secret in another. The tragic heroine in one plot may then be the vengeful demon summoner in another.
It's not that originality is overrated or that I am urging you to jettison such ambitions, but if you, like me, have been paralysed before by how it feels everything has been done already, it's okay. Sometimes that's not a bad thing.
3) It has renewed my appreciation for active characters.
As a game organiser, it is a lot easier to easier to entertain proactive characters (and players) than passive ones. And there is nothing quite like knowing someone will be playing that character for around 48 hours to make one realise how little agency they have and how little they have to do.
Live roleplay characters need concrete goals and long term plans all rooted in clear motivations. Otherwise it is very easy for a player to get bored. Everyone needs to have a detailed backstory and an articulate inner life. The stakes may differ as one character may have their life on the line and another might simply be trying to futher the ambitions of their family, but everyone needs have something to do and a reason to do it.
This is, to an extent, less true for fictional characters as they can always have the plot hot on their heels chasing them. Think the reporter who has seen too much, the innocent who has been marked by the monster or the heroine who has been chosen by fate.
But in live roleplay, plots requiring another character to be the instigator tend to backfire. There is just no guarantee that the other player character will act on the plot and if it hinges on a nonplayer character, it is entirely possible for the plot to fall down the back of the sofa and be forgotten. Live roleplay characters who just wants a quiet life can find that it is an ambition a little too easy to fulfil. Telling them to keep their head down is only fun if the site can be plastered with wanted posters and an angry persecuting mob is actually hot on their heels. A character who has reason to run towards the danger rather than away from it is just easier to entertain.
4) Gameworld "Levers" are an interesting way to conceptualised agency.
In a live roleplay game, character agency is created when the things a character desires to do is supported by the gameworld. They then pull with what is commonly called a "lever", that is to say they undertake an action that has effects on the setting that is beyond the immediate. This can be them personally passing legislation as a senator, but it can also be them manipulating/bribing/convincing a senator. The flavour text may change but it is a way of thinking about what actions a character can undertake.
A character may refuse to use a lever and that itself is also power of a sort, as long as they know they could have and that they are choosing those consequences. Failure, especially in live roleplay, can be a form of freedom. Knowing that it is possible to lose can make winning sweeter. The importance for a player is that these are foregone conclusions. We all know that the referees probably won't end the world if there are another three sessions left, after all, but when it comes to fate of this province or whether or not the shadow butterflies would go extinct, things that can go either way, then there is a more tangible threat and a solidity of choice.
Levers are very much about exerting conscious power. They aren't about unknowingly releasing ancient evils or accidentally torching libraries. The roleplay is less focused on the difficulty of the action and more in the decision and the reasons to act. They are also thus meant to be broadly predictable in their outcomes, though complications are no bad thing. This may seem counter to plotting methods that centre around try/fail cycles, but I'm holding it as example of what pure "character agency" looks like.
5) Characters Can Always Surprise You.
I have written hundreds of plots and dozens upon dozens of characters, but I have never been able to actually guess how the characters would behave once I put them in the hands of the players. For all that give them backstory and motivations, schemes in progress and conspiracies to enact, I can never quite predict the outcome. I always liken writing an event to setting up a field of dominos, but unlike Hevesh5's gorgeous domino setups mine do ridiculous and unknown things. Characters with impossible plans pull them off and others are toppled from their seemingly invincible position of strength. I remember when one skin-stealing monstrosity came out to his squirrel girlfriend only to realise that she had concluded that it was fine since they were both monsters. Compromises are found in unexpected quarters, heroes fold under pressure and villains find that last shred of dignity. Kidnapped heirs will negotiate and free themselves, a stray arrow slays the promising young senator and the nonsense plan to raise a new pantheon of gods somehow succeeds. The game organisers' lair is always full of astonished whispers of "wait, they did what now?!"
Capturing that feeling of surprise and spontaneity in one's own writing can be hard. Ask anyone who has lost the interest of a friend when trying to explain the intricate happenings at their last roleplay session. And even in recounting these stories in part here that it seems all too cliche, all too possible. It was Pratchett who joked that million to one chances are far better than certainties on the Disc.
But it is all about that sense of possibility and agency for roleplayers, that their characters are in their hands, that they can indeed do anything and anything can happen.
 No link has been provided because I am intimately familiar with how easily it is to lose hours on that site.
 It has become common in the format to ask to play characters who are homage (aka blatant rip offs) of ones from pop culture. However what one might consider to be the "core" of a character differs from person to person, so in recent years we have begun to ask players to describe what they want to play without naming it. Do they enjoy the being a know-it-all apect of Holmes, the being a damaged genius with a doctor best friend or the solving intricate mysteries part? Or even having a nemesis known as the Napoleon of Crime?
 It varies from game to game, of course.
 But some have, if only to prove the point that it is possible. And I have nothing but respect for that.