Targeting champions under turrets

So, what happens if you hide under a turret so that your champion cannot be clicked? You can still auto with the A+left click

You can hit them with skill shots by aiming

Some targeted spells work on the nearest champion to where your mouse is pointing. But some targeted abilities don't work if you can't actually click on them, right? Or am I wrong? For example, with or without smart cast, if an enemy champ was already under the turret then Kha'zix six went in to Q the champion, he would be unable to because you can't click on the enemy champion, right? Or does it just not work without smart cast

If you could give other examples that would be helpful