The Common Sense Rule
Facilitation of arbitration.

The one rule that should trump all others, to preserve smooth game flow in a player-friendly game: use common sense. You can’t be expected to always know every rule for every situation. If you know a rule exists and you know exactly where to find it, then sure, look it up. Or if it’s a life-or-death situation, allow the player any chance available to save their character. But otherwise, keep it moving. Arbitrate, announce your decision and move along. Check later.

Don’t shut down player imagination. Players will often have their characters attempt bizarre and unexpected actions. Allow heroic feats based on circumstance, but don’t necessarily let them become a rule. Explain the scene clearly with a hint at the odds. Sometimes the result is not what the player imagined. Narrate why it fails or succeeds so the player can feel that you are adjudicating fairly.

Encourage use of the environment for physical actions, and all manner of variables for social encounters. Grant a bonus or penalty of plus or minus 1 to 5 to a D20 roll, based on how compelling the player’s description of their character’s action is. If, in character, they present a persuasive, well-worded argument, they should be rewarded with a bonus to a diplomacy check. If they announce that they are focusing their attention on a relevant area, they should receive a bonus to their perception check.

Allow rolls even when the rules would not. If a character does not have the skill officially required for a task, but has related, indirectly relevant skills, or background traits that suggest relevant knowledge, allow them a chance. Give them a penalty to the check if necessary, but let them try.

For potentially complex encounters, consider requiring extra rolls for success. One single diplomacy roll to avoid hostility should not necessarily equate to the multitude of initiative, attack, damage and saving throw rolls that might be required to overcome an encounter through combat. Require rolls based on circumstance. For example, three separate diplomacy or bluff checks covering an introduction, argument and conclusion might be needed for a character to talk their way through a dangerous situation.

Of course, a roll is not always necessary. Unless they are in direct danger, or peril would result from failure, allow characters to take 10 on any reasonably straightforward skill check that they have ranks in. Even just making a correct choice can clearly dictate a character’s success in some situations. Keep it moving. Use the time saved to add vivid description and engaging interaction to further bring your world to life.