Rigid thinking can be cast only upon a creature with Intelligence of 3 or greater. The creature is allowed a saving throw to avoid the effects.
The creature affected by rigid thinking is in capable of performing any action other than the activity he is involved in when the spell takes effect. The creature's mind simply cannot decide on another course of action—it becomes frozen into a single thought and cannot change even if new circumstances would suggest otherwise. Thus, a warrior fighting a kobold will ignore the arrival of a beholder, and a thief picking a lock will pay no heed to the arrival of three guards.
The affected creature does not mechanically repeat the action; he is not an automaton.
He will not continue to fire his bow at a dragon if he runs out of arrows, but will choose another means of attacking the dragon to the exclusion of all other activities.
A spellcaster in the process of casting a spell when rigid thinking takes effect will not attempt to repeat the spell (unless the spell has been memorized more than once). The spellcaster will, however, devote his attention to the target of that spell until his goal is met (e.g., if the caster were attacking a creature, he would continue to direct attacks at that creature; if the caster were trying to open a door, he would continue to work on the door until it opens).
The spell expires when the creature accomplishes his goal (i.e., the kobold is killed or the lock is opened) or when the duration of the spell has ended.