Hiding is not just lounging behind the barrel, merely trying not to jump up and shout. It involves controlling your movements and breathing and, when the hiding place is good enough, keeping an eye out for the people you are trying to hide from. Simply ducking and burying your face in the sand might not use up activity, but that is not a very good course of action. I'd say that actively hiding is perhaps even more than a 20% activity.
Another factor I was looking for in this thread was that preparing a spell not only involves speech, somatics and perhaps material components, but you do those things to gather magical energies that accumulate at your hands or head (mentalism) to shoot away as a spell when you cast the magical energies.
I know RAW doesn't state it specifically, but I'm using such cinematic effects for my spell casters. Casting a fire bolt would see small tendrils of flame gathering to the caster, then those tendrils form a small ball between his hands and at casting the ball shoots forth as a 1" bolt, streaking across the battlefield in a fiery arc.
For channeling casters I have bright lights spring up from the ground, or golden energy gathering or choirs singing in the background and mentalism users project halos or get brightly glowing eyes and such.
This is tied, in part, to what type of spell is being cast and it is more pronouncing in F and E spells and more subtle in U and I spells.