The level of interaction between players on Discworld is controlled by the status of 'player killer'. By default players are not player killers which means that other players cannot kill them, steal from them, loot their corpse or place contracts on them at the various assassin guilds.
Players who wish to do these things must apply to be player killers using the 'apply' command. Once registered they will be able to do these things to (and have these things done to them by) other player killers.
Once you have decided to be a player killer, there is no turning back. To emphasise this again, there is no turning back ever.
Player Killers are commonly referred to as PK. In opposition, those who are Non-Player Killers are referred to as NPK.
Many people are worried about becoming a playerkiller due to the perception that weak characters will be mercilessly victimised, but in general this is not the case.
Yes, you will be stolen from. If you carry items of sentimental value or that are hard to replace, you may want to either stay NPK or consider vaulting them. But in general, you won't be stolen from that much, and most thieves are willing to sell things back to you or even just give them back if you explain why the object holds such value to you. Obviously this doesn't apply if the object in question was taken because they were trying to hurt you, but if you're reasonable and polite, you shouldn't have a problem.
The main annoyance is that if you have low perception, you basically can't carry coins. Go to the bank when you want to buy something and don't idle in public places with cash on you and you'll be fine.
This will happen, but not as often as many fear. Spend your xp frequently as a precaution, and if someone attacks you, take it with good grace. Random attacks can be a fun part of the game, either trying to get away from your opponent before they kill you, or turning the tables on them. Or in more even match-ups, using various skills and tricks to try to come out on top.
This happens even less often than random attacks, due to the cost. Most people won't bother with more than warning contracts, because those are much cheaper, and even real death contracts aren't much of a problem due to the coded time limit on the frequency of contracting. Make sure you have a few lives spare and try not to repeatedly annoy people with a lot of cash and you'll barely even know assassins exist.
For an action to be pk-checked generally means that playerkillers can do it only to other playerkillers, and non-playerkillers can do it only to NPCs. Broadly, this applies to:
- Attacking, stealing from, or looting their corpse without being permitted
- Casting offensive spells on
- Performing offensive rituals on
- Picking locks on real estate owned by
- Things that interfere with covert actions (the Light ritual, spraying perfume at)
There are other, more specific, things that are checked. These include (but may not be limited to):
- The Calm and Chant rituals. These may, however, be performed on players who are grouped with you.
- The Celestial Anchor ritual
Some actions have a less strict check:
- Find Corpse can be used on npks, and can be used on pks after being "permit"ted
- Tempt is only checked for poisoned fruit
- Track can be used on npks
There is also something called a "pk assistance check", which means that npks can't perform the action on pks who are in combat with other pks. It applies to the scrying systems.