Autocast abilities proc under certain circumstances. Slow procs once the enemy right clicks an enemy. If the enemy is an ally to the unit, which has the Slow ability, it will cast it. It also works upon attack.
Others, such as Heal, work once a unit enters X range from the caster. If it is injured, the ability will be automatically casted.
So, up to the aim of yours, pick up the right ability. An autocast ability that meets no standards, but has a very limited amount of options to edit are Frost Arrows/ Searing Arrows/ Poison Arrows.