Skill Checks
A skill check is made to perform some problematic action that requires skill. There are twenty skills in the game, and each one has a governing statistic. A skill check is a d20 plus the character stat modifier (always), proficiency modifier (if the skill is trained), and miscellaneous bonuses (which can come from various sources). Characters can attempt to use non-intellect-based skills they are not proficient in. They still roll a d20, but only add their governing stat modifier (not their proficiency modifier). Intellect-based skills are different; they cannot be attempted untrained.
Success Levels. When a character attempts a skill check they roll a d20 and add their total modifier, attempting to hit the target difficulty. If they roll a natural 1 the roll is a critical failure and something bad happens. If they succeed by 5 or more the roll is a critical success. If they succeed by 10 or more the roll is an epic success. Whenever a skill check has the potential for some additional benefit beyond simple success or failure, these levels of success can come into play.
Cross Training. Sometimes the same activity can be performed using different skills. There are many examples of this; Acrobat and Athlete can be used to jump, Survival and Nature can both be used to predict weather, Investigation and Diplomacy can both be used to summarize a pub crawl looking for rumors, etc. In situations where two skills could both be used to perform the same task, it is appropriate to assign different difficulty levels for each if one is more applicable than the other. Toolset training may also be used in place of a skill if the task directly relates to the toolset being used.
Support. Sometimes a character making a skill check can benefit from the support of someone else with the same skill. In situations where this applies, the supporting character grants a +2 bonus.
Teamwork. Sometimes everyone in the party needs to attempt the same skill check. In these situations, at least HALF of the characters must succeed for the group to succeed. The pros are assumed to adequately support their less able companions and get them through it. This is particularly true in the case of athletics, deception, and stealth checks. If half of the party can get up the wall, trick the guards, or sneak up on the orcs, everyone can.
Negotiation
When interacting with NPCs, players must often negotiate for what they want. The core social skills are:
Diplomacy. A combination of persuasion, argument, and negotiation.
Perform. Covers different forms of entertainment and inspiring emotion.
Empathy. Involves reading tone and body language to sense emotion or intent.
Subterfuge. Covers lying, tricking people, planting ideas, and impersonation.
Intimidate. Inspiring fear or paranoia by various means, usually to gain compliance.
Because roleplaying is a major element of GSS, these negotiations can and should take the form of a conversation. But this has the side effect of rewarding well-spoken players with success, regardless of their character's charisma score and social skills. Characters with good social skills may also say inappropriate things or offer arguments that make no sense, expecting their statistics and die rolls to persevere. This problem plagues all role-playing games, and no simple solution exists.
But here is the method in GSS:
The Storyteller must first establish exactly what the players try to persuade their target(s) to do. No social roll should be required if their argument is sensible and benefits the target without serious risk. If their argument requires risk or sacrifice on the target’s part and is unnecessary, then a roll must be made regardless of how well-spoken the players are. Such a roll also cannot be made unless the Storyteller believes the argument should have a logical chance of success. If the request is ridiculous or the consequences are too severe, do not allow a roll, regardless of how persuasive the characters are.
Also, note that social skill checks may have gradients of success. Even if a social roll was not high enough to justify whatever the players wanted, a respectable roll might result in a lesser form of support or compliance instead.