In a Pathfinder game, when a spell caster lobs an area of effect spell into a group of creatures with spell resistance (SR) and the spell is affected by SR, do you roll your caster level check once, or must you make the check vs every creature with SR?
You make the check v.s. every creature with SR, but you might (check with DM) only roll once. If you beat an individual creature, that creature is affected. If you don't, only that single creature is protected against the effects of the spell.
Spell resistance applies if the resistant creature is within the spell's area. It protects the resistant creature without affecting the spell itself.
and, more relevantly:
To affect a creature that has spell resistance, a spellcaster must make a caster level check (1d20 + caster level) at least equal to the creature's spell resistance.
In general, this and the general structure of the section on spell resistance would seem to imply you follow the process for each creature and thus roll once for each. The following one line, however implies otherwise:
The defender's spell resistance is like an Armor Class against magical attacks.
All vs-AC attacks that hit an area, like the X-laser, shotgun, and automatic weaponry, roll once for all creatures in the field of fire.
Neither of these implications are explicit and both lines of reasoning are valid, RAW. Rolling once per creature will consistently have mixed results, while once for the group will be much more swingy. Generally swingy is bad, but in this case it's also much faster for large groups of enemies.
Quoting myself for the 3.5e version of this question, since D&D 3.5e and Pathfinder use exactly the same rules text for spell resistance:
If your spell is being resisted by a creature with spell resistance, you must make a caster level check (1d20 + caster level) at least equal to the creature’s spell resistance for the spell to affect that creature.
This very much describes a pair-wise situation: a creature resisting a spell. You are rolling a check against that creature’s SR. If there is another creature also resisting that spell? Roll a separate check against their SR. (Note that Pathfinder uses absolutely identical language for this.)
As a result, rolling just once per spell is a houserule, probably intended to speed up gameplay.
That SR is “like” AC is completely irrelevant to this discussion: “like” does not mean “behaves exactly the same as for all purposes and obeys every rule of,” it’s just a description that there is a conceptual similarity between the two, which may be helpful in understanding how it works and what it’s for.