The PC is in the encounter as soon as the situation is described; he sees the ogre menacing the family.
The player can choose how the PC will respond to the situation presented: he can try to negotiate, engage in social combat, physical combat, or attempt to use an Aspect in the environment to help resolve (if a Fate point is available). You don't get a Fate point for facing a situation.
Or, he could walk away. You get a Fate point for facing a situation when you otherwise would not have, except that you have an Aspect that says you should.
The Player is out of Fate points. The character has several stress boxes checked, and a consequence from a previous fight. Overall, the character is kinda battered, and a rational person would be trying to get out of the dungeon to rest. The character stumbles into the room, seeking an exit...
He sees the ogre and the family. He *could* just turn and walk away. He does not have to respond to the situation at all!
The player says, "Screw this, I don't want to get more beat up, I'm leaving!" And the GM holds up a Fate Point and says, "But you're a Defender of the Innocent..." That's a compel.
If the player says, "Crap. My character's beat up. A sane man would walk away. GM, I'm a Defender of the Innocent, do I get a Fate point if I engage?" That's the player suggesting a compel.
Mind you, this is on the edge of being a double-compel, as the character really is beat up, and didn't have a choice. If you're going to *force* a character into a life threatening situation, giving two points might be warranted.
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Now, make it a giant snake, instead of an ogre.
The player says, "Screw this, I'm leaving!" The GM says, "But you're a Defender of the Innocent" and holds up a Fate point (or two, if the GM feels this is risky enough to be a double).
The player, having no Fate Points left, doesn't really have a choice in the matter. He gets a Fate point and steps forward.
The fact that it is a snake is not yet a complication! The player is still free to deal with the snake in any way he pleases. The situation is not yet more complicated for the character than the ogre would have been. The fight may actually run for a little while normally...
The GM now says, "Why did it have to be a snake? You're beat up and didn't want to get into this fight, and you're not winning fast. Your fear is rising, and you find you have to keep your distance..." and holds up *another* Fate Point.
The player can now either keep his distance (and have two Fate points to be creative with) or buy off that compel with the earlier Fate point and attack normally.
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This partially demonstrates how "Why did it have to be snakes?" is not a great aspect to have. We think, "He has that aspect, so facing a snake is risky for him, and is thus a complication!" That's because it is written to seem only negative - we don't see how the player can turn that Aspect into an asset. But, if the player is creative, he can turn that around...
The player is compelled to fight the snake, and has been given one Fate point. He's not yet been compelled to keep his distance.
The player says, "Why'd it have to be a *snake*? My character has had a hard day. He's been knocked around, abused, insulted, and all. He's fed up. He looks a the serpent and thinks, 'Insult on top of injury? A snake? Really? REALLY?!?! ARRRGH!' and flies into a rage and attacks." Passing the Fate point back to the GM, he rolls dice, and adds two to the roll for his righteous anger.
So, now we see that it being a snake can be used by the player, making the thing easier, not more complicated.
Thus, the snake isn't a complication until it restricts the players action, and not before.