1) Is a player expected to narrate how each trait in their pool is influencing their action?
Up to the group. The description can be short but clearly be informed by the traits used, or the player can flat out state why they are using some/all of the traits, or whatever you prefer. It's probably easiest to get in the habit of stating why each trait is used at first, so players get used to the dice pool building (and the subsequent tweaking that comes from SFX, plot point uses, and other players/GM pitching in ideas), but after a while certain things may be very obvious (i.e. including Strength to grapple someone).
2) Is declaring a particular trait set to be a prime set basically announcing that the table is going to be pretty forgiving about whether a particular trait applies or not? I mean, one is generally expected to take a trait from a prime set on every roll.
Like, what exactly is the difference between having Relationships as a prime set vs. just having them around but not prime?
Not forgiving, no. More like, a Prime Trait set is important, and any Prime Sets should be used on every single roll, with very few or no exceptions. So announcing Relationships are Prime isn't about saying "yeah, okay, you can use a relationship on this roll" but rather it's saying "you
must use a relationship with every roll." In those scenarios, if there is no relevant relationship, you'd still add a die representing one, likely a d4 or d6 to represent a fleeting, inconsequential relationship (until PP or Growth are used to make it permanent, of course).
If you absolutely cannot justify using a Prime Set on a roll, then you should think "Should this set be a prime set?" first, "Do I even need to roll the dice here?" second, and then after that, decide "How do I make up for that missing die on this particular roll?"
3) Is there any point in having both Signature Assets and Resources in a particular game, or should it be one or the other?
Depends on what you're trying to achieve. I've certainly used both in a given campaign sometimes, but not every time.
Resources notably work different from other traits, so you should really understand those differences. You "commit" dice from Resources, and add them to get the Total above and beyond the usual "two dice added together make the total." Then those committed dice are spent for the session, lowering (or removing) your Resources until the next session (or some other recovery milestone).
4) Likewise, is there any point to using both Challenges and Crisis Pools in a given game? Or is that basically a genre decision?
Same as above. Depends. I haven't used both in a campaign, but I also haven't played much since ToX came out, so I haven't seen the actual in-play differences between Challenges, Crisis Pools, and Mobs. There are some subtle but important differences in how they all work. Notably, all of them are tools, so it's (eventually) worthwhile to understand them all to be able to use them when they make the most sense. As a beginner, though, stick with one for a few sessions and just play and have fun. See what works. Cortex won't break, believe me.
5) There's three basic ways to add to a Growth pool: Questioning trait statements, being healed of stress or conditions, and achieving goals. Will growth be noticeably faster if one includes all three, rather than the first two that the Handbook assumes?
Thanks for your time, guys!
Probably. Cortex doesn't have a big "zero to hero" progression in its system, which is why it's so tight and mechanically works so well. It's very bounded, but fiction-first so that you can set the dial of how gritty-->cinematic things are. But with advancement, racking up dice increases and adding new traits a bunch really doesn't do as much as you think. At their "best," characters can only have so many traits, and so many of them ranked at d12, and so many SFX before it becomes the "worst" for their player, which is option paralysis. If their character has 25 relationships all ranked at the same rating, what's the point?
A general piece of Cortex advice is that it's okay to tweak what traits are in play and what mods you use as you go. It's hard to "break" the game, but it
is a different paradigm for the players, so changing trait sets and what's prime and what's not might feel like it's going to be problematic and mess with the player's expectations, but it really shouldn't. You just have to be prepared for it, which means having a conversation with the players ahead of time that this stuff might occur. Cortex actually really works well as a system that's almost like it's considered in "playtest" for the first several sessions, and as you all get used to the setting and characters (and most importantly, the types of conflicts that crop up), you might tweak things here and there as you go until you settle on the thing that works.
On the same hand, don't just change stuff out constantly. Let things stick around for a few scenarios and sessions, really test them out, and only after doing so a few times, consider what (if anything) needs to be changed, swapped out, or whatever. As much as the game fully works in "playtest" mode, it can be boring or frustrating if the player's stats or the GMs use of Doom Pool vs. Static Difficulty vs. Crisis Pools etc. changes every single session, or worse in the middle of a session, especially if you didn't give it an honest few shots.
