GURPS characters are often faced with temptation. They find themselves in situations where what they want to do is not the same as what they (or their player) knows they should do. That may range from the frankly sensible, like not sticking your head out to join in a firefight because you’re scared of being killed; to the self-indulgently foolish, like allowing yourself to be seduced by someone you suspect may be trying to rob or blackmail you.
Obviously, as a player running that character, you can choose the ‘right’ approach every time. Your character will join in the firefight – after all, it won’t really hurt if he’s killed. And he won’t be seduced by the wrong people.
Unfortunately, life’s not like that. People do the ‘wrong’ thing all the time, and GURPS has two mechanisms to enforce that: will rolls, and self-control numbers.
Some people are determined. They go to the gym regularly and exercise hard when they are there, even though they don't enjoy it. In GURPS terms, they have a high Will trait. Others are less able to motivate themselves. They join the gym as part of their New Year's Resolutions to get fit, but drop it after one or two visits. They have a low Will. (Those with a really low Will keep paying their membership because they can’t face the phone call to cancel it!)
Will rolls indicate a character’s ability to resist psychological pressure, whether that be torture, social pressure like the ‘need’ to wear trendy clothes at school, or even resisting temptations like seduction when they feel they are being set up.
Another form of psychological pressure to which Will rolls apply is Fright Checks – a character coming upon a frightening scene may be stunned, throw up, or develop a stress disorder depending on how badly the Fright Check is failed.
GURPS treats a character's Will as being equal to their Intelligence (IQ), unless the player has chosen to raise or lower Will with character points. That's a game-mechanic to avoid over-complicating the character generation system: in real life, Nobel prize-winning physicists are probably not noticeably more likely to resist torture than ‘normal’ folks.
In some cases, the temptation comes from the character's particular mental disadvantages. Someone who may normally be very determined and self-controlled can have a ‘blind spot’ where a particular temptation affects them more strongly than most people.
Take smoking for example: many people you would think of as determined have tried many times to give up smoking but failed. GURPS treats the addiction to tobacco as a disadvantage: it treats the addict's chance of resisting temptation differently to their normal Will roll because they are so much less able to control themselves about tobacco than the other things in their lives.
Addictions aren't the only things which call for self-control rolls. Many aspects of life can be expressed in terms of temptation, and self-control rolls apply. Someone with a fear of heights, for example, will need to make a self-control roll to climb down a ladder to escape a burning building – the comparative risks may be clear to the outsider, but to the acrophobic, the chance of finding a place to hide till the fire is extinguished can seem to be a smaller risk than the ladder.
That's the essence of the self-control number: the ability to make decisions about your actions based on what you know is good for you, rather than what's immediately desirable. The lower the self-control number, the greater the likelihood that the character will give in to that particular temptation.
Will rolls and self-control numbers can be required in situations anybody can find themselves in. We all know that feeling where you are tempted to finish the pie instead of wrapping it up and putting it in the fridge. What makes some people wrap up the pie, and others reach for a second or third helping? Which is appropriate – a Will roll or a Self-Control roll?
The answer is that self-control rolls are only required where the character has a specific related disadvantage. A Glutton will make a Self-Control roll, everyone else relies on Will.
The Third Edition of GURPS applied the Will roll (sometimes modified) to disadvantages. For severe disadvantages (worth double the points) the character had to take a -4 modifier to their Will relating to that disadvantage. A disadvantage always cost the same regardless of how high or low your Will was, and you could not be good at resisting one temptation and bad at another.
In Compendium I, that was changed with optional rules that decoupled Will from IQ – more realistic, but they acknowledged that it would involve more book-keeping. It also introduced the concept of the Frequency of Submission: the ability to vary the cost of a disadvantage depending on how easily you could resist it. That is now the standard rule for 4th Edition (page B121), and all Disadvantages are 'bought' with a Self-Control number.
Divorcing self-control numbers from Will meant that an easy-to-resist disadvantage doesn't give you as many points back, allowing you to tailor them to your character concept.
A player may voluntarily fail either Will or Self-Control rolls. It may seem perverse, but it's all part of good roleplaying. Some players will take disadvantages to gain the extra character points and always attempt to resist them. Others will see the disadvantages as part of their character concept, and will only try to resist them if there is some really important reason to do so.
As an example, a recent character of mine had the Pyromania disadvantage in a time-travel game. We were in 18th-century London, there were riots going on, and we had heard rumours of widespread arson. We had to secure and protect someone who was in jeopardy. Fine, we went ahead and did it, rescuing him from the bad guys and heading for our 'safe house' lodgings. We saw a mob in a side-street ransacking a magistrate's house. Clearly the GM expected me to try to make a self-control roll to continue with the party to the safe house. Actually, I decided that (as far as my character Jim saw it) the action for the night was over, the VIP secured and on his way to a safehouse, and the villains would take time to get over the setback, regroup and find us. Jim thought he had time to have some fun, so I didn't even make the roll – Jim just slipped away and joined the mob, looking for a good stick and some rags to make a torch!
A good GM should reward good roleplaying like this and penalise those who are always trying to resist their temptations.