Rushed

Rush:

You can interrupt initiative and act before your turn, at a penalty. If you rush another characters turn this takes the place of your action and you can have a free, move or combat action at the penalty recorded below.

A character Rushing as a way to interrupt another character's action only gets 1 action that turn.  A character using the Rush rule can opt to take an additional action on their turn. Rushed or additional "Free" or "Move" actions that typically don't include checks now must have a average check that, if failed, fumbles your whole action. Attack or skill actions used with Rush have a -20% (-4 in d20, +2 to difficulty in White Wolf/U5D) penalty. A failed Rushed action costs the character's next turn.

This can be a existing combat rule (optional in the Universal 5 Dot concept), an ability (for GURPS), or a D20/Pathfinder Feat. Rush can be taken as a fighter's bonus feat. Characters and creatures that get multiple actions a turn can counter a rush with a parry, dodge or counterattack as an action without penalty. Rushed actions cannot benefit from modifiers about aim or concentration. Rushed actions are affected normally by weapon and envromental modifiers. In the case of multiple characters rushing on the same phase, only the first to declare their rush acts. If there is a question you can roll initiative, but rush actions should always be quick. Players should have their action immediately described, it would be fair to cancel the action (but not the penalty) for a player that slows the game with rushed actions.

Rush cannot be used with spells (see the Metamagic feat Quicken Spell). For other settings any mental or psychic special ability would also be incompatible with Rush. Physical special abilities would be compatible. A character can rush the final turn of a multi-turn non-combat action to have it activate before the turn they interrupted as long as they accept the penalty to the roll. Game Masters may also consider other limitations if Rush is used too often. Perhaps it can only be used once per encounter, or at the top of the round. If multiple characters have or can use Rush, the GM may decide that only one character can use Rush in a turn.

Notes:

NPCs generally shouldn't use Rush unless there is a compelling story reason. The GM already has many tools for adjusting NPC initiative and Rush can easily feel like a personal attack if it is used against the players. GMs should be careful when introducing Rush and should be open to applying additional penalties to prevent it's overuse. Rush was designed for high mortality short battle skirmish games, such as modern settings where a single gunshot can be fatal to even a seasoned character. However I feel that it is easily useful in most settings.