I react at the list building stage to a bunch of potential inputs, including scenario, expected terrain, and things I perceive to be strong or popular (which can include 'what's is being played by people who I consider to be very strong players that I expect will be attending' when we're talking about the runup to an event). I know there's an old drumbeat of 'it's not your list', but really, your list should contain a selection of tools that let you engage with the challenges you're likely to face in any given game. Often, though, I'll find that I can adapt to a new challenge without significant list changes, provided the list had a robust set of tools already. Usually this is a case of putting a few mental cycles into it - for example, if I'm on the way to the game store and have some spare headspace, I'll ask myself questions like like 'without changing my current list, how would I engage a limited insertion Marut list? How would I engage a twenty order galweigan or ISS horde? How would I engage a Kamau? How would I engage a linked heavy infantry missile launcher?' Almost always an answer presents itself after a bit of working through the problem. Sometimes it doesn't, and in that case that can be cause for a bit of a list tweak, or more often think about the pair I'm running.