Tricky Spells (Su): Starting at 5th level, an arcane trickster can cast her spells without their somatic or verbal components, as if using the Still Spell and Silent Spell feats. Spells cast using this ability do not increase in spell level or casting time. She can use this ability 3 times per day at 5th level and one additional time per every two levels thereafter, to a maximum of 5 times per day at 9th level. The arcane trickster decides to use this ability at the time of casting.
So what exactly does it take for someone to "notice" a spell? I've assumed that verbal and somatic components make it obvious that someone is casting a spell; so lack of them would make spellcasting... not obvious?
If a 5th level Arcane Trickster were to walk up to the king of the realm and cast a Trickified Dominate Person on him, would anyone notice? If not, what if the king made his Will save? Or what if his advisor were an Archmage with permanent Greater Arcane Sight or Detect Magic at-will?
Any previous rulings on this?
There's nothing official that I'm aware of. The Spellcraft skill states you must be able to "see" the spell clearly, but that would mean that you'd only need Still Spell to hide casting completely. That, and Verbal only spells are unidentifiable.
I've always read "see the spell as it's being cast" as "perceive the components of the spell as it's being cast". Therefore you still have a chance to ID a Stilled Silent spell if it uses a material or focus component that you can percieve. That's strictly my interpretation though. I also tend to think there should be cumulative penalties for each component removed, but that's a little more in depth and I haven't thought about it in detail.
Casting a Silent Still spell with no material components still provokes an attack of opportunity. So there's still something noticeable going on, even if it's not obvious.
Perhaps the caster should make a Concentration check (Casting Defensively DC) to continue moving and talking normally while the spell is executing.
After that, the caster should probably need to make a Bluff check to avoid looking like he's trying to project mind beams into the King's head.
Of course, all this could probably be obviated by casting the spell while hidden or invisible. Which an arcane trickster is likely able to accomplish, I'd think.