Dr. Awkward said:
A True Neutral character is "disinterested," or "noncommittal". He doesn't go out of his way to be good, nor does he go out of his way to be evil. He doesn't care much about structure or freedom. He mostly wants to do what he needs to, without ethics muddying the picture. He'll shrug off a good character's admonitions for lacking altruism just as he'll shrug off an evil character's admonitions for being a soft-hearted fool.
Saying that one action means that the character has chosen a side of the fence implies that one step off the path of your alignment means your alignment changes. A True Neutral character sometimes does good, and sometimes does evil, sometimes upholds law, and sometimes chaos, but he doesn't do these on the basis of principle, but because he feels that the actions are demanded by the current situation.
A True Neutral fighter might kill an innocent person to protect himself if he thinks that the person will report him to the enemy, and later on protect a group of innocents from his town from the depredations of a group of bandits. He's not particularly cruel or merciful, but in order to be Neutral he doesn't need to refrain from being cruel or merciful either. He might join a militia and follow orders like a good soldier, and then later desert when he figures out that his commander is an idiot. He's not particularly lawful or chaotic, but doesn't need to refrain from acting in either way. Eventually, it will balance out somewhere in the middle.