Line of effect vs line of sight

[Ozreth]

So if you want to shoot a ray of frost 20 feet across the room at an orc, but your friend is directly in between you and the orc, you can't cast the spell (well you can, but youre friend gets hit). Does it work like this with every spell? What about shouting at your friend to duck?

And what about shooting an arrow? I've not found anything that talks about a percentage chance that you may hit your friend or whatnot.

How do you guys handle these things? We've always just avoided the situation but it's been coming up a bit lately.

[Jacque]

I believe the orc would get cover, if an enemy or ally is in between.

Also remember the -4 penalty for shooting a ray of frost or an arrow into a melee fight, unless you have the precise shot feat.

[Kumori]

Cover
One of the best defenses available is cover. By taking cover behind a tree, a wall, the side of a wagon, or the battlements of a castle, you can protect yourself from attacks, especially ranged attacks, and also from being spotted.
[indent]To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).

This passage shows how to deal with ranged attacks when something or someone is in the way. It doesn't reflect any chance of the attack actually hitting the thing, but the target does get some protection.
If you're doing something that doesn't need an attack roll, either it doesn't matter that something is in the way (single target spells, for example), or it's gonna hit both of them by going the first (ala Lightning spell and other 'line' effects).

Edit: Precise shot only helps when the thing in the middle is adjacent to the target and is your friend. If it's an enemy there's no -4 penalty for shooting into a melee because you're not trying to not hit him by accident.

[Ozreth]

So:

Shoot an arrow, friend in middle is safe and orc gets cover.
Call Lightning, friend is fine, orc gets zapped.
Shoot Magic Missile, friend gets hit and the missile stops OR friend gets hit and the missile continues on to the orc (who also has cover?).

[Kumori]

Shoot an arrow, friend in middle is safe and orc gets cover.
Yes.

Call Lightning, friend is fine, orc gets zapped.
Yes.

Shoot Magic Missile, friend gets hit and the missile stops OR friend gets hit and the missile continues on to the orc (who also has cover?)
Magic Missile would just hit the orc, same as call lightning.

My example above that would hit the friend is a spell with the 'Line' descriptor in the 'area' portion. Those go through things.

I realize now that I said Lightning, but I meant Lightning Bolt. That was an error on my part which may have caused confusion.

[Feytalist]

Rays are ranged touch attacks, which is essentially the same as normal ranged attacks, so things like cover and shooting into melee counts. Magic missile and any other "target" spells (such as charm person, hold person etc.) require no attack roll, so cover does not matter. You must still be able to see the target though, so total cover or concealment would interfere.

Spells like lightning bolt also require no attack roll, but they affect everything in their AoE (a straight line, in this instance), including your ally, should he be in the way. Call lightning (and flamestrike) are special cases, where the point of origin is above the target rather than in your square, so theres nothing to worry about, unless there's a flying creature right above your target.