Zulkir of Dartmouth said:What does QED stand for?
http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=qed
Therefore you can assume that an archer with no spell training or spellcraft ranks would ASSUME that when he sees a spell caster cast they would do something, like move or speak. If the archer does not see this he would be hard pressed to ID if a spell is being cast.
Why? When you come right down to it, casting a spell basically drawing power from (wherever a spell's power comes from) and channeling it into whatever form the caster wants. This is clearly an act that takes a significant effort on the caster's part, and distracts him at least momentarily (provokes AoOs, requires Concentration checks if disrupted, etc).
So it's not a trivial thing to cast a spell. An archer may not necessarily know that the spellcaster is casting any given spell, but if he sees the caster "zone out" for a moment, chances are he's going to figure _something_ is up. And what's the most logical thing to happen, if it's a spellcaster doing things? They're most likely casting a spell. Hence he fires his arrow.
To take a real world example, I would like to say that I am watching a clown and I am going to shoot the clown when he attempts to blow up a balloon. If the clown has a special ability that allows him to blow up his balloon with out moving or blowing air into the balloon how am I going to tell when he starts blowing up this balloon?
Still Spell doesn't mean you cast a spell without moving. It means you cast a spell _without somatic components_. You still need to concentrate, and this can be betrayed by your actions. To use your analogy, think of the clown as having a special ability to blow up the balloon by pressing a remote control that fires a gun. He doesn't have to aim the gun (Still Spell), but he still has to press the remote control (concentrate). This can be a clue for an alert observer.
In the end, though, thinking too hard about fantasy is bad. Rule it how you like. Trying to get too deep will just get you into all sorts of contradictions.
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