- All spells other than cantrips have a minimum casting time of 1 round, unless they previously required a swift/immediate action. At the start of the next round, rather than releasing the spell you may choose to extend its casting time by another round. ... For every round in which you maintain a spell in this manner you take 1 point of nonlethal damage per spell level.
- If you take damage while casting a spell you lose the spell automatically, unless that damage is backlash from the spell itself. Casting defensively does not exist.
- If you are adjacent to the target of a spell as it is being cast you may attempt to gain its effects as well as a free action, similar to the Spell Stowaway feat. This requires an unarmed attack roll opposed by the caster's caster level check. For instance, when an enemy spellcaster teleports away you could attempt to teleport to the same location along with him. Whether or not you succeed, you take 1d4 points of damage per level of the spell. The caster level for the stolen effect is equal to the original caster level or your BAB, whichever is lower. The spellcaster does not gain any knowledge of whether you are successful. You cannot attempt this on the same casting more than once.
A couple of thoughts:
What is the point of holding a spell rather than casting immediately? You take (nonlethal damage), open yourself up to interruption for a longer period of time, and get no benefit unless you're waiting for something to happen, and you just wanted to start casting because you weren't sure when that thing would happen.
Cantrips that deal damage become stupidly useful against other spellcasters and spellcaster v. spellcaster fights turn into standoffs. What I mean is that, since cantrips maintain their lower casting time, they can be used to deliver the damage needed to make the opposing spellcaster "lose the spell automatically." Neither spellcaster will want to start casting a non-cantrip spell because the other will simply knock them out of it.
Additionally, though I recognize this may be intended, these rules make spellcasters highly dependent on other players in ways that non-casters are not dependent. By this I mean the casters must have someone to prevent the interruption, so they need the meatshield or whatever. The meatshield, while less deadly, does not rely on the spellcaster in the same way. This also makes solo spellcasters almost useless unless they have a lot of downtime to prepare defenses or are using other "tricks" (admittedly there are many). The interruption mechanic is very harsh. The casting time already dramatically changes effectiveness.
Another observation: the spellcaster, if adjacent to the target, can steal the effect for himself. This is odd, though not necessarily bad. However, should you want to cut it off, I would suggest not disabling all other actions while casting (to prevent the use of the free action to steal the spell) as that would prevent such mundane things as talking
(a free action).
Lastly, what happens to spells with irregular cast times, such as swift action castings? There are entire spells devoted to that (Swift Haste, for example), as well as other spells that, while less overt about it being their point, depend on such cast times (one I can't locate at the moment, was a direct damage spell from, I believe, the retired Spellbook article).