I've noticed that when engaging enemies from behind cover, if you're patient enough in the 3d camera mode, there are times when certain alien types will pop their head out from cover. If I wait for them to peek out, and there is at least 40% to hit you I have hit almost everytime with a crit, resulting in death.
Half cover reduces chance to hit by 20%. Full cover reduces chance to hit by 40%. Factors which influence chance to hit are displayed when you hit F1 while you are selecting a target.
Animations of the 3d models are not listed in that screen and do not influence chance to hit.