Sometimes you want to be sneaky and try to cast a spell while noone is paying attention. This is particularly important for spells like Charm person, Suggestion, etc that you may want to use in a non-combat situation.
Normally spell casting is rather dramatic and is fairly obvious with verbal, somantic and material components. It is as obvious as whipping out a longsword. But as with drawing a weapon you may wnt to be mroe subtle about it. In this case use the following rules:
First, make a concentration roll. DC = 10+ spell level. This allows you to focus on casting while doing it in a manner you are not used to and while trying to speak the words quietly, keep the gestures small, etc. If the caster is also trying to do other things like hold a conversation, walk, etc then a penalty of -1 to -5 should be assessed (per GM)
Then you need to make a bluff roll to disguise the spell. This is opposed by others Sense Motive skill. If spot or listen is more appropriate (such as trying to hear a spell in pitch blackness) the DM can choose to substitute either skill.
The bluff roll gets the following bonuses and penalties:
+2 (each) for the spell not having a verbal or somantic component or if the verbal or somantic component cannot be seen or heard. (such as if the person is deafened and cannot hear you speaking or you are casting a spell in the dark and they cannot see you)
+1 for the spell not having a material component.
If the person attempting to detect the spellcasting is distracted the GM can award a bonus of +1 to +5 to the caster.
If the spell has a focus then you get a -1 to the roll.
If the spell has an obvious requirement (such as crushing a gem, eating a live bug, etc) then you get a -1 on the roll (or higher per GM).
If the person you are trying to fool is paying direct attention to you (such as watching you to the exclusion of others, etc) then -2 to the roll.
The GM can assign other bonuses or penalties as the situation warrants.
Note: Subtle Spellcasting does nothing to cover the actual spell effect. It just helps hide the casting of the spell.