There will be many times when you will be required to roll a skill check to determine whether your character was successful in an attempt to do something. Attributes (such as Strength or Intelligence, described in the Attribute Descriptions section of Character Creation) are really just built-in skills that everyone has, and checks against them are handled identically to checks against other skills, so, for brevity, we just refer to both types of rolls here as skill checks.
Whenever your character uses a skill, a difficulty and modifiers may be assigned by the GM. Sample difficulties and modifiers are listed in the skill descriptions where applicable.
Difficulties are typically positive numbers, and are based on how difficult the action being undertaken is, compared to what is considered "normal" for the skill.
Modifiers may be either negative (making the attempt more difficult) or positive (making the attempt easier), and reflect any unusual conditions which are present at the time of the action. Modifiers are often assigned based on a lack (negative modifier) or excess (positive modifier) of equipment, although this is only one of many possibilities.
Another common modifier is the effect of successfully making a skill check in a complementary skill. If your GM decides that a previous success in one skill check would affect the current skill check, you might be given the amount that the previous roll succeeded by, as a positive modifier. Whether or not to apply a failed roll as a negative modifier is likewise left up to the GM, but this point should be clarified before making the first skill check, to avoid unpleasant surprises. (An example of this is given in the Disguise and Con Artist skill descriptions).
Unlike some game systems, all difficulties and modifiers are applied to your character's skill level, not to your roll. Rolls are made on a d% against an "effective" skill level, which is determined by starting with the skill level, subtracting difficulties, and adding modifiers. A positive difficulty means that the effective skill level is reduced, making success harder, while a negative difficulty makes it easier to accomplish the task. Conversely, a positive modifier makes things easier, and a negative modifier makes it harder.
Sil (with a skill level of 62 in Acrobatics) attempts a single flip off of a pommel horse. This is a pretty simple operation, so the GM assigns a difficulty of 5. There is a spotter, and the GM decides this will be worth a modifier of +10, so her effective skill level for this flip is 67(=62-5+10). If the horse was slippery, the GM might assign an added modifier of -15, for an effective skill level of 52(=62-5+10-15).
All difficulties and modifiers are supposedly determined by the GM as a result of a careful examination of the situation, but time and game flow may not always allow this. If you think that a difficulty or modifier is unfair, discuss it. The GM does not necessarily know everything about your character, and may have overlooked something. Try to keep these discussions to a minimum, however, as they can draw things out and cause lots of aggravation. The GM should be trying to keep the modifiers applied for a particular situation fairly consistent, and may even want to keep a list of the ones which are used most often.
There are two basic situations which you may find your character in, and attempt to use a skill to determine the outcome of.
In the first case, it is your character against someone or something, not in direct competition. In this case, after all modifiers have been applied, you will roll a d%. If your roll is less than or equal to your character's effective skill level, the attempt succeeded. Otherwise, your character's biggest worry may be when rigor mortis will set in.
In the second case, your character is participating in a competition of some sort, be it a race, a game of skill, or whatever. In this case, all modifiers will be determined for all participants, and each will roll a d%. If at least one participant's check was successful, the winner will be the character who succeeded by the largest margin. If everyone failed their check, and a winner is required by the nature of the contest, the winner is the one who failed by the lowest amount. If there is a tie (by either method), it may be broken by awarding the win to the character with the higher skill level, or it may be considered a tie, and another contest held to determine the champion. The exact nature of the resolution depends on the nature of the competition.
In keeping with the heroic aura which typically surrounds fictional characters (including characters in movies, books, and, yes, role-playing games), an effective skill level cannot go below 4. If, after all difficulties and modifiers (including modifications from the use of Luck points, described in the Attribute Descriptions section of Character Creation) have been applied, the effective skill level is less than 4, it will be immediately increased to 4, no matter how crazy the deed attempted is. This is to represent the "unbelievable luck" factor which often makes fiction more interesting than real life. Anything your character tries, no matter how crazy, has a 1 in 20 chance (0-4 on a d%) of success.
Unfortunately, there is also such a thing as bad luck. The effective skill level cannot go above 94, so that any roll of 95-99 fails automatically.
There are times when a roll will be so good (or so bad) that something spectacular may happen. Generally, any roll which falls into the guaranteed success range (0-4) is a critical success (including rolls where the effective skill level actually was 4), and any roll in the guaranteed failure range (95-99) is a critical failure.
It is up to the GM to decide exactly what the effects will be. We suggest that the first two numbers in each category (03, 04 or 95, 96) would be one step above a normal success/failure, the next two numbers (01, 02 or 97, 98) would be two steps above, and the extremes (00 or 99) would be three steps. Exactly what a "step" represents depends on the situation and is therefore left to the GM to determine.
For example, in combat, a roll of 03 or 04 would cause maximum damage to be inflicted, or allow the player to choose the location where the blow landed. A roll of 01 or 02 would cause maximum damage and allow the player to choose the hit location. A roll of 00 would cause double maximum damage, and allow the player's choice of hit location.