You roll the dice when there’s some kind of interesting opposition keeping you from achieving your goals. If there’s no interesting opposition, you just accomplish whatever you say you’re trying to do.
As said previously, characters in a Fate game solve their problems proactively. Players, during the game you’re going to do a lot—you might break into the bad guy’s fortress, pilot a starship past a minefield, rally a group of people into a protest, or poll a network of informants to get the latest word on the street.
Whenever you take action, there’s a good chance that something or someone is going to be in your way. It wouldn’t be an interesting story if the bad guy just rolled over and handed you victory on a plate—clearly, he’s got some crazy security measures to keep you out of his place. Or the mines are unstable and already blowing up around you. Or the protesters are really scared of the cops. Or someone’s been bribing the informants to keep quiet.
That’s when it’s time to take out the dice.
Cynere needs to bribe her way past the guards keeping her from entering the city of Thaalar. Amanda says she’ll do this as a straight-up overcome action, because the guards are nameless NPCs anyway and not really worth a conflict .
Lily looks through Cynere’s skill list and picks Resources as her skill, hoping she can scrounge enough out of her coin purse to satisfy them. Her Resources skill is Average (+1), so she’ll add one to whatever result she gets from rolling the dice.
She rolls and gets: +-0+
Her total result is +2 (+1 from her dice and +1 from her skill of Average), which corresponds to a Fair on the ladder.
As said in The Basics, whenever you roll the dice, you’re comparing your roll to your opposition. Opposition is either active, meaning it’s another person rolling dice against you, or passive, meaning that it’s just a set rating on the ladder which represents the influence of the environment or situation you’re in. GMs, it’s your job to decide what the most reasonable source of opposition is.
Amanda decides to roll active opposition against Lily on behalf of the guards. She decides the most appropriate opposing skill is Will—they’re trying to resist the temptation of bribery, after all.
The guards are nameless NPCs with no reason to be particularly strong of will, so she gives them a Mediocre (+0). She rolls and gets: ++0+
…for an incredibly lucky result of +3!
That gives her a Good (+3) result, beating Lily’s roll by one.
If a PC or a named NPC can reasonably interfere with whatever the action is, then you should give them the opportunity to roll active opposition. This does not count as an action for the opposing character; it’s just a basic property of resolving actions. In other words, a player doesn’t have to do anything special to earn the right to actively oppose an action, as long as the character is present and can interfere. If there’s any doubt, having an appropriate situation aspect helps justify why a character gets to actively oppose someone else.
If there is no character in the way, then look at your situation aspects in this scene to see if any of them justify some sort of obstacle, or consider the circumstances (like rough terrain, a complex lock, time running out, a situational complication, etc.). If something sounds interesting, choose passive opposition and set a rating on the ladder.
Sometimes you’re going to run into edge cases, where something inanimate seems like it should provide active opposition (like an automated gun) or an NPC can’t provide proactive resistance (like if they’re unaware of what the PC is doing). Follow your gut—use the type of opposition that fits the circumstances or makes the scene more interesting.