When a PC in combat casts a spell when they come up in the initiative order, people ARE where they ARE. If you can see it and it's in range of the spell, you can target it and there's NO chance of hitting something else unless the description of the spell specifically notes it. The bit in the Fireball description about "passing through narrow areas" is a somewhat more exotic application of the spell than simply targetting someone on the other side of a crowded battlefield. Casting a spell so that it passes through a small hole is different than simply making it explode on someone's face, mostly since the intention is to have the spell detonate in an area you CAN'T actually see instead of where you can see. The bit of description about hitting things that may be intervening between the caster and the target has a LOT more to do with things like Walls of Force and Invisible creatures than it does for normal stone walls and creatures that you CAN see.
Yes, this is a gross simplification but it's the way the rules work, and it's a simplification that avoids having to resolve all kinds of annoying inconsistencies (such as lobbing fireballs or anything else past large intervening opponents who might otherwise be considered to be moving.
Remember that one does not "aim" a fireball - it's actually targetted like any other spell. It has the particular drawback of BEING ABLE to impact on something prior to detonating at its targetted location, but that doesn't mean that as a DM you should be looking for ways to re-interpret and interfere with it simply functioning normally every freakin' time it gets cast. That ISN'T what the "pre-detonation" thing is there for unless you're doing that with every freakin' spell in the book - in which case you'll get the angry, frustrated players you deserve. The spell should not go awry unless you can't see (by accident or intent) where it's going to impact. If it were intended that the spell COULD be so routinely and easily interfered with then it wouldn't be providing rules for sending it through arrow slits - IT WOULD BE PROVIDING RULES FOR JUST GETTING IT PAST EVERY CREATURE ON THE FIELD SO IT COULD EVEN BEGIN TO BE USEFUL.
The game is designed such that what you want to hit with a spell, you hit; where you want a spell effect to go, it goes. AFTER the spell gets to where you wanted it to go, or hits the person you wanted it to hit is when you generally start dealing with avoiding the effects by a last minute dodge, resisting the damage dealt, etc.
As always, the right of a DM to use his own preferred interpretations and house rules cannot go without saying, but there is no need to start hanging additional rules on Fireballs and how they work. Fireball is really a rather straight-forward spell.
Now it is true that in "reality" the NPCs are still moving when the PC casts his spell. But for all rules purposes, except those for which specific provision is made (firing into melee, AoO...), neither their past or future movement, nor their current activity bears additional consideration for resolving the spells effects. For example, in reality the person being targetted by the fireball in question may be moving at a dead run - but it doesn't matter where he started that movement, if he will continue that movement, nor how far or fast he's moved. It doesn't matter what anyone elses movement is, was, or will be. The only thing that matters is where he is NOW because on a given characters turn all actions are resolved on that basis - where things are now.
As for attempting something like "swatting a fireball out of the air as it travels to its point of detonation", yes it's theoretically possible - but I wouldn't even BEGIN to allow anyone to think to try it unless they had ability to snatch/deflect arrows and recognize spells as they're being cast, as those would be the minimum sorts of skills that would be needed to attempt it. It's a move that is definitely exotic and thus has no rules to support it. It REQUIRES the DM to adjudicate the situation and that has several requisites of its own. Any individual trying to just hit a speeding fireball with his hand and making it detonate early must,
A) identify the spell being cast as Fireball (or guess correctly that it will be if you're going to ready an action to accomplish this little feat)
B) have the ability to act out of initiative order if you're doing this on the fly
C) have some kind of feat or ability like catching/deflecting missiles as the action being taken or readied
D) have the fireball whizzing through your occupied space unless whatever feat/abilities you're using enables something other than that.
I can see this being done by a monk, ninja, dwarven defender, or multiclass mage if they're EXPECTING a fireball as the casters next spell, but this has to be a wild, bizzare tactic for characters with particular appropriate skills and NOT for the average mook who is in the line of fire.
That what Im REALLY after. *grin*
