What are some ways to help a GM who is running an adventure you've written to improvise the rest of the adventure if a PC takes an unexpected action?
Truly a remarkable example of cluttered thinking at it's very, very best. Not only do we have GMs who are put in the position of having to control the game participants when the adventure is broken by players daring to show independence, now we have a game designer who wants to help the GM when the situation arises.
My answer? Stop thinking you can control everything.
For years I've been reading DMs on Reddit and elsewhere posing situations and asking questions about players "breaking the game" by doing something outside the adventure's schematic. It wouldn't be hard to see this as a real problem of games, something along the lines of a catholic minister asking his bishop, "What do I do when the parishioners start thinking for themselves," or a rapist asking, "What do I do when the victim won't lie still?"
No action that a player takes in a campaign should ever be "unexpected." That's my philosophy. Players should always be expected to act in a manner that is free from outside control and influence, on their own initiative, in a manner that is allowed to be pursued autonomously and which happens in accordance with the players' desires. The only curb on player behaviour should be the natural consequences that arise from the level of civilized society that has been impressed on the players as existing at the time of their actions, so that the players can decide ~ for themselves ~ if they can avoid or handle those consequences.
Those consequences should be obvious to everyone participating. If I murder someone in an RPG in an open street, whether it is during the Roman Empire, the Middle Ages, the 19th century or the era of Buck Rogers in the 25th century, I and everyone I'm playing the game with ought to be shrewdly aware that a) someone is going to witness it; b) witnesses are going to talk to the authorities; c) the authorities are going to seek redress to the extent of their abilities; and d) that redress is going to depend on how powerful the authorities are.
All of that has fuck all to do with whatever adventure is happening or what the DM had planned for the next session. It could be that some element of the adventure that was in the DM's mind might be affected by the murder, but again, that effect ought to be completely logical to everyone playing. If it is not, if the DM is adjusting for some dumbfuck adventure that was written by someone else, then the DM has a head up an ass someplace.
Improvising is fine. The action I took, killing someone in an open street, just started a whole new adventure, one which I obviously preferred to have when I made the conscious decision to commit murder. And the DM ought to have considered long in advance that someday a player would do such a thing. How? Because we live in a world where people kill people every day, and it is hardly a huge shock or surprise that it happens. Maybe not in front of us, I'll grant. But given that the players of a game are carrying weapons all the time, and know how to use them, and have in the past, it isn't exactly a long stretch to figuring out that the guy in the street isn't exactly inviolable.
I hate when DMs make up their minds what my player will or won't do, simply because they want to be "set managers" of the tiny, tiny theatre they've concocted in their imaginations as the whole of acceptable human experience in an RPG. And I equally hate players who happen agree to exist in such a tiny, tiny theatre.
Thank you, I'll embrace the whole human experience. As it happens, I probably wouldn't commit an open murder. I played that adventure a lot when I was 15 and 16 and it's somewhat juvenile now. My capacity for doing the "unexpected" is, I'll have the reader understand, pretty fucking extensive ... so a DM with me in the campaign better buckle in and expect everything.
I don't play because I don't find DMs like that.
Except in a mirror.