mmmm - Sigil... got a few problems. I had to struggle with this thing myself - and actually had to reverse part of My opinion on the spell a few games ago.
1.) If a spell is cast and the caster or target is in an AM field, the spell automatically fails (for the purposes of area effects, the center of the area is what is checked).
No - the spell *works* - but its effect is supressed - in a similar fashion to how a Vampire's command/domination ability used on an individual inside a magic circle. That btw - is the precedent for considering how "supression" works. This DOES effectively "counter" instantaneous effects - but not something like charm person.
2.) If an area effect is centered outside an AM field but would include the AM field in its range, the effect stops at the edge of the AM field as though it hit a wall of force.
mmmm - I would drop the "as though it hit a wall of force part". See the illustration below.
3.) If the center of an ongoing area effect comes under the influence of an AM field, the entire effect is supressed.
No - only the portion of the effect inside an AMF would be stopped. See the illustration at the end of this message.
4.) The durations of spells "supressed" in an AM field continue to "tick down." IOW, if I have three rounds left on a spell and step into an AM field for four rounds, the spell has already expired when I step back out. It does not "pause" the countdown - and then continue for three more rounds when I step back out .
yep
5.) An AM field does not block a line of effect between two points that are both outside of the AM field (my so-called "quantum casting" effect). This is implicit in statement 1, by the way.
Actually - this is implicit in statement 4; and the fact that the spell is merely "supressed" and not dispelled upon impacting the field. The fireball bead or what not is "supressed" as it passes THROUGH the field. This can be somewhat important if you are using meteor swarm.
Now - for an illustration.
Lisa; the good wizard - has an AMF field with a 10'radius in place. Her familiar, Pax, is 15 feet away from her. Zorus - the Evil Archmage - not realizing the AMF is in effect, casts Fireball directly at Lisa.
Q: What happens?
Pax is struck by the fireball - and Lisa is unaffected. Why? Only the area of the spell that overlaps the AMF has it's effect supressed. In this case - that is the center of the effect. Any area outside the AMF is still affected normally.
Q2: Can Lisa cast a spell at Zorus with her AMF still up?
A: yes
Counterargument:
Likewise, it (AMF) prevents the functioning of ... spells within its confines.
It is possible to argue that all spells, spell like abilities, and supernatual abilities have as a function their "activation" - and in the case of "spells".
However, The PHB glossary definition of spell is "a one time magical effect" that is separate and independent of its means of activation, As "one time magical effects" need not of necessity be "cast".
A "spell" which falls into either the general category "Arcane" or "Divine" does need to be "cast", but this is a method of activation and
only an invocation of "Rule 0" can render the act of casting as part of the function of an arcane or divine spell.
Q3: Can Lisa use a scroll or wand to cast a spell at Zorus with AMF still up.
A: No, because activation is a "function" of a magic item, and the spell prevents their "functioning".
Does antimagic field actually prevent the casting of a spell from within the field? That is, does the spell automatically fail, and is wasted?
No. An antimagic field suppresses magic used within, brought into, or cast into it's area but is not dispelled (see the second paragraph of the spell description).
First of all, the second paragraph is not what may prevent the "functioning" of spells. It is the first paragraph, and that requires forcing "casting" to be a "function" of an arcane or divine spell.
As far as play balance and creating a prison of some kind for a spellcaster.
Blinding (or blindfolding) a spellcaster renders it such that they have no "line of effect" beyond their person - or theoretically a touch spell. Manacles can prevent them from touching something you don't want them to touch (and the blindfold gives them a miss chance as well.)
A dimensional anchor effect prevents teleportation or escape by extra-planar means. IMC - this just means that the walls and door have to be lead lined - or made of lead, and is non-magically possible.
The "hard part" is the fact that spellcaster can still place a spell on themselves. Polymorph self and similar magic can be incredibly helpful in facilitating an escape - but require an EXTREMELY inventive individual.
Furthermore, the use of poison or magic that drops the individual's mental attribute score to 9 or below can prevent spellcasting, and effectively reder them a "normal" prisoner.
Now - if you have magic and are willing to use it - the following work quite nciely
1. Feeblemind (but this does make them vegtables... not necessarily a desireable or "good" thing)
3. Mark of Justice - set to trigger upon the attempted use of magic - with a curse effect that lowers the character's spellcasting score. This works - provided the spellcasting stat is below 15 and you keep magic items away from the character. If the character has a stat that is a wee bit too high - it could be combined with the drugging effects mentioned previously.
3. A Geas - worded as such - "Thou shalt not attempt to use magic in any shape, way, or form".
An attempt to use a spell then causes 3d6 damage - forcing a concentration check. Furthermore this "penalty" could occur on each breach of the geas; depending on how the GM rules.
4. Lesser geas, Geas, or a Mark of Justice compelling an individual to remain within the prison; also work quite well - causing debilitation if the individual attempts to escape.
If the individual were capable of escaping your prison despite all of these precautions - the extreme measure of a binding spell (if available) would work on a far more permanent basis. Similarly extreme is outright killing the individual - and then using trap the soul.
When it comes to restraining a powerful magic user... where there is a will to do so - there IS a way to do it. You just have to think hard enough to do so.