Hiya!
I was worried about this before it started and it's actually worse than I feared. My worry was that, with an entire town of NPCs to pester, the PCs would be constantly talking to anyone and everyone making planning for an upcoming session quite hard.
Whell THAR'S yer problim!
I'll make it short: Don't "plan"..."prepare". HUGE difference.
Plan: "NPC ABC knows about NPC XYZ, and NPC LMN is in cahoots with XYZ. When the PC's confront ABC, LMN will not be mentioned".
Prepare: "The 'Cabal' is focusing on keeping the PC's in the dark about LMN".
The difference is that in the "planned" version, you are assuming that the PC's
will not know about the ABC/LMN connection. If one of the PC's uses a spell to get more info, or an astute Player puts two and two together...well...everything you 'planned' for that session relying on the PC's ignorance is FUBAR. But...if you go the 'prepare' route, you may have certain ideas/things going on in the background that you can use and modify that would 'make sense' if the PC's go and confront ABC.
The worse part is that they (reasonably) keep expecting the powers that be to sort out their problems. In the most recent session, one of the party got taken prisoner by the local gang and after discussing possible options amongst themselves they went with going to the city guard to help them recover their comrade. Absolutely reasonable and, of course, no self-respecting city guard person is going to ignore a cry for help (and it doesn't help that one of the PCs was a member of the city watch, so has contacts). But they're doing it a lot and it's a bit frustrating.
Again... the difference between "planning" and "preparing". If you are planning...then yeah, the law/guards getting involved is likely not 'planned for' in regards to encounters and whatnot. If you 'wing it' for one encounter, that will mess up something in the next...which will quickly snowball to the point where the 5th to 14th encounters make zero sense (if they are even possible). But if you had just 'prepared' then you know the make up of the guards, the make up of the thieves guild, the amount of training/skill each 'side' has, where their headquarters is, who is likely to support each side if a 'street war' breaks out, have maps of barracks, warehouses, hovels, market places, and so on, as well as a few pages with typical NPC's stats (commoners, 'tough guy', children, dogs, etc), etc.
Now I certainly didn't help myself by having them going up against a gang that is much more powerful than them (they're level 3) but instead of trying the softly, softly, sneaky, sneaky approach they went running to the guard for backup (again entirely reasonably to choose that option, but it turns them into bystanders). Amusing anecdote, they tried this a couple of sessions ago when they discovered the identity of a villain (and left a mess in the process) and then while they were off rounding up the constabulary, the villain returned discovered that their lair had been ransacked and made their escape, and is now plotting their revenge... I thought that might have learned them...)
This is exactly the thing you should be doing!
I bet you didn't "plan" for the villain to get away and plot a revenge, did you? That would be part of "preparing". Ask yourself "Ok, what IF he has a change to escape? What would he likely do, based on his personality, goals, alignment, etc?"...then, no matter what the PC's actually do, you are "prepared" for it.
So (and I realize I'm opening myself up to a lot of criticism here) what, if anything, can I do about this? And is this the norm for urban adventures where NPC aid is around pretty much every corner?
As I said...don't PLAN....in stead PREPARE.
The easiest way to do this is, IME, to
not design encounters based on the PC's make-up (re: their individual classes, races, what spells they have, magic items, etc). All you need is amount of PC's and their average level. After that, who cares. It's not your place to pre-determine how an encounter is going to go or if it is even going to happen. That's ENTIRELY on your Players. For example...when they went to the constabulary. Did you 'plan' for the possibility of the PC's trying to force/blackmail/threaten the captain of the watch? I'm betting no, you didn't. If you HAD...would you have made him a 'tough fight' to a group of 3rd level PC's? BAD DM! NO! NO!
You look at the size of the city, the danger level, and how 'tough and experienced' a guard would have to be to become Captain of such a city. Lets say that works out in your mind to be "8th level". Well, if you were
preparing, you'd have that info. If the PC's attack...well...not your problem. The PC's all die or get arrested. Is this a problem? Yes...IF you were
planning stuff....and an absolute No if you were
prepared. Would such a turn of events ruin the adventure? Again, Yes...if you wrote it with a "planning for the PC's capabilities/actions"...and, again, an absolute No if you wrote it with a "preparing for the PC's to be in the city".
Bottom line: It's not up to you to make sure the PC's win (or even CAN win). It's up to you to portray a living, breathing, believable and consistent fantasy campaign world...where NPC's and groups all have their own goals in 'life'. They are not there to be "reasonable challenges for the PC's to mow down".
Then again, I guess I qualify as a "killer DM" by today's standards... so there is that.
^_^
Paul L. Ming