While I've never written a LARP, one of the subtler points about writing them that I've noticed from playing is that it is difficult to make plots that require people to be clever in character. The basic difficulty is that if I'm presented with a challenge to which I (the player) see an obvious solution (or even just first step towards a solution), but my character sheet doesn't even mention this obvious idea, I will assume that for some reason it is not obvious to my character. If my character sheet says "you hope to figure out a way to do X" and there's an obvious way to do X, in character it doesn't make sense for me to be "figuring out a way" if the obvious way was obvious to my character. So if you're writing a character and part of achieving that character's goals involves coming up with a solution to a problem, you need to make sure the solution is difficult enough (or inaccessible with the knowledge that the character starts out with at the beginning of the game) and that you don't conspicuously leave out any obvious ideas the character should have for going about the solution.
I should mention that not all (perhaps even not most) people play characters this way, which sometimes bothers me (and causes my characters to be much less successful than others'). For instance, in a game I played in recently, one of the tasks of a group of characters was to design a budget, which involved raising some money and apportioning it in a way that would receive approval from a majority of the group. Everyone's character sheets just said things like, "you want to fund this, you don't want to fund that, you don't want to raise money this way". There was a very obvious solution to the budgeting process, which was to write down all the things you cared to fund and compare it to the total funds available and see how much you needed to raise (in fact, you didn't need to raise much money at all, even if you wanted to fund almost everything that anyone wanted to fund). Nothing was said about this (or the results of it) in the character sheets, so as I played my character, this is something that wouldn't happen until everyone sat down and started debating the budget (and at that point it would actually happen pretty quickly, because it is kind of the only way to do it). However, one of the players decided to do this ahead of time, and came into the game with a grand budget plan to present to the group, most points of which were fairly quickly accepted. While in some ways this is something that would have made some sense for their character, this is something that I would never do if my character sheet didn't say to do it. Seriously, if you have some budget priorities and you know how much money you have to spend and your character hasn't already done the obvious calculation and thought about it (with the outcome and your thoughts about it mentioned in the character sheet), that seems to me to be a nontrivial statement about your character.
Anyways, the moral of the story is that character sheets need to be very thorough, because an omission of an idea in a character sheet is just as significant as an inclusion.