The novels are a bit inconsitent on this subject, and the rules are a bit vague. (Or nonexistent?) Therefore I pull the following out of nowhere in an attempt to capture the spirit of the latter portions of the series, purely as a house rule (this actually derives from a previous discussion found at
http://www.jimbutcheronline.com/bb/index.php?action=post;topic=30822.15;last_msg=1306527):
Create the following hypothetical new stuntlike power:
Empower Magic Circle [-1] The character adds the Empower Magical Circle trapping to their Lore skill. This trapping allows the character to make a Lore declaration to change an existing scene aspect such as "Circle drawn in chalk" into a special scene aspect such as "Magic Circle drawn in chalk" -- and have it be true. [Optionally, the GM might decide that the character can spend one mental stress to cause the circle to be treated as a threshold with a rating of half the character's Conviction (round up, minimum 1)].
Discourage anyone from actually buying this power/stunt (except spellcasting characters, who are considered to have it for free), but allow it to be 'borrowed' in accordance with the Temporary Powers rules (YS92) by spending a Fate point. Once used, the scene aspect is treated as a 'fact' that triggers compels against the high concept of creatures that are affected by magic circles as described below.
- Purely supernatural creatures (ghosts, demons, etc) cannot break the circle; this is treated as a compel against their high concept if the circle prevents them from acting in accordance with their nature/goals.
- Other non-mortal creatures with material bodies (vampires, etc) are reluctant to break the circle; treat this as a compel as above, but the creature can buy off the compel.
- Mortal creatures with or without supernatural powers can break the circle freely (no compel).
If any creature breaks the circle (by penetrating the barrier, either personally or with an object), the circle is disrupted and the scene aspect is removed. However, if circle was disrupted by a creature possessing supernatural (or magical) powers, their powers are temporarily weakened: attach the aspect "Violated a Circle" to the creature to represent this. This aspect can be tagged/invoked for a bonus
against any use of the creature's powers (for example, resistance against spells cast by the affected creature or attempts to injure a creature with supernatural armor). Those inside the circle can break it freely with no adverse effects (other than the circle breaking).
Note that all of this is a long-winded way of saying that the player can do something to make a special declaration (justifing a creature-appropriate high-concept compel) and perform a scene maneuver (that conditionally penalizes powers) for the cost of a Fate point and an action.