At what point during spellcasting does failure from invalid targets occur?

[Brissan]

Let's say you try to cast a spell where the target is invalid. At what point does the spell fail in the below situations? Does the spell not even start, or is it expended without any actual effect? When a spell fails for this reason, is it obvious to the caster why it failed, or does it seem to be the same as if the target succeeded their saving throw?

1) Undead disguised as a living human, targeted by Charm Person
2) Any spell attempting to target an area beyond the line-of-effect blockage of an invisible Wall of Force

[bbangerter]

1) The spell is expended to no effect.
2) The spell is expended. Effect is determined by the type of spell. Spells that do damage will damage the Wall of Force as normal. Most spells will simply fizzle.

As for the caster knowing why, that's going to be more a GM call. For 1) I'd only tell the caster the spell doesn't take hold (they can assume a save was made).
For 2) I'd probably tell them something blocked the spell at point X. For damaging spells it would be visually obvious they were blocked.

[gnoams]

Targeting happens when you finish casting a spell (mentioned under aiming a spell in the crb).

For the other questions, I don't know of any actually spelled out rules anywhere.

I've always played it as you get invalid target when you try to aim a spell at the wrong thing. You could instead say, they appear to be immune or something along those lines. If you're feeling gracious, it'd be perfectly reasonable to allow the caster to choose a different target. If you're feeling mean, I've seen some GMs say you have to pick a valid target for a spell. So if you cast something that targets a humanoid, you have to pick a humanoid for the spell to effect, and if there aren't any enemies in range, well I guess you have to chose one of your friends to target then.