The rules as written are a bit ambiguous on this. There are several instances where the Complication Range expands “on its own”, e.g. by damage to systems or space pheonomena nearby.
But how do I, as a GM, raise the Complication Range? Page 279 of the Core Rulebook indicates that an increased Complication Range is (or rather: can be), in itself, a Complication. The GM can create a Complication by spending Threat (cf. p. 281).
So… if an increased Complication Range is not set with the scene but introduced later – does the GM have to pay 2 Threat per increase above the Complication Range already set?
Or is, on the contrary, the Complication Range determined like setting a Difficulty, more or less arbitrarily set by the GM (p. 83 can be read as such)?
It can be a bit of both to be honest. Pure RAW complication range is meant to be increased by traits. As such spending threat is always an option to you. But as you are crafting task rolls on the fly, you are also able to bake in traits (whether you call them out or not) that also increase the range.
If someone is using a rope bridge to cross a ravine for example: If they describe themselves as dancing their way across, when you build the task roll I would add in a trait relating to their lack of caution. Their approach may be giving them access to a different attribute to roll, but its also affecting the risk of the task. On the same check you could decide to add in that one of the guide ropes is frayed. You spend threat to add the complication if youre deciding that on the fly, or just have it written in if you’d decided this in advance.
My understanding is this:
It can be inherent to the situation, caused by circumstances, or you can buy the increase.
An example of it being inherent: O’Brien is trying to fix a conduit on DS9 during an attack by the Jem’Hadar. Unfortunately, it’s close to a critical system and so he is likely to set off the anti-intruder systems. The complication is inherent to the situation, and is represented by the increase in complication range.
An example of it being bought: O’Brien is trying to fix a conduit on DS9 during an attack by the Jem’Hadar. The GM decides that it’s time to make things more difficult, and so spends threat and a Jem’Hadar squad turns up and stations themselves nearby - if O’Brien makes too much noise, they’ll hear him and he’ll have a fight on his hands. This risk is represented by an increase in the complication range.
You can do it either way. I often allow the circumstances to dictate the details of the role (like complication range, giving advantage, etc). You do have to be careful with stacking though, and so be mindful of how difficult it will be and perhaps ease off the threat if it’s too hard - or find excuses to give them bonuses.