The Hazards of Invisibility, or,
    Making Your NPCs Respond the Way They're Supposed to
                       by C.E. Forman

Writing "The Path to Fortune," my first serious attempt at IF, has
taught me a lot about puzzle design, but one realization has stuck
with me like no other, and I thought it might save some new
authors a lot of trouble if I passed it along. Namely, I learned
to never, ever, allow an object which alters the NPCs' reactions
toward the player to find its way into the mainstream scope of an
open-design game.

Here's what happened: One of the items the player acquires in PTF
is a cloak, which, when worn, makes its user invisible. This is
necessary (obviously) for solving one of the game's puzzles.
Unfortunately, neither I nor my co-author gave it much thought at
design time.  Only after I'd sat down and tried to program it did
I realize the possibilities involved.

PTF's design makes it a very open game. That is, players have the
ability to explore about 80 percent of the game world right from
the start -- only a small number of locations are blocked off by
puzzles.  PTF has 11 major NPCs, and about a dozen minor creatures
for the player to interact with. Almost all of these are
accessible from the start, and there's no limit to what the player
can take along when visiting them.

Starting to see what I'd gotten myself into?

The player has the ability to wear the cloak almost anywhere in
the game, because there is no means of limiting its transport. No
logical method of restricting the cloak's location came to mind,
and re- design of the entire game was out of the question -- I'd
already been programming for three months when this came up. (It's
amazing what you think of when you actually sit down and attempt
to code something.)

Now, since the player can wear the cloak anywhere, this means it
can be put on and taken off in front of the various NPCs, and can
also be worn when the player walks into the location. What this
meant was that I had to go through all my NPC objects and
interactions, adding evaluations that checked if the player was
wearing the cloak (since, for example, you shouldn't be able to
give something to a character who can't even see you).

Compounding the problem is the fact that the game by itself was
already fairly complex to begin with. A particular spell can be
used to put characters to sleep, the characters have their own
waking and sleeping cycles, and a couple characters' behavior
changes drastically during the course of the game. All this meant
more checks and confirmations. The simple act of adding that cloak
easily quadrupled the complexity of the NPC objects' code.

In the end, I ended up modifying most NPC actions with a check for
the cloak, but left some of the less changeable alone (such as
killing characters while wearing the cloak -- I couldn't very well
have the player be able to walk away from something like this, as
it would alter the state of the game far too drastically). For
some of the more magical or more alert characters, I simply
decided to let the player interact with them in the usual manner
(although this did occasionally require mentioning the cloak,
which meant that an 'if' expression was still in order).

I'm still finding corrections that need to be made because of the
cloak. No doubt beta-testing will reveal even more that I missed.

The moral of the story is this: If, when designing a game, you're
considering the addition of an object that: 1) Can be used more
than once in the game; 2) For whatever reason, makes the NPCs
behave differently around the player; and 3) Exists in a game in
which most of the locations are accessible by the player, at any
given time, and while carrying the aforementioned object, do
yourself a favor and think it over several times before adding it.
Redesign the puzzles or map while you still can. Don't rush the
initial design phase -- take your time, thinking everything
through, and try to see every possibility from the outset. Ensure
that coding every possibility is worth the trouble before you
start. Hopefully it'll save you from some of the trouble I ended
up going through.
                              - o -