I find myself asking players roll Deception when they try to intentionally mislead an NPC, even if it is a lie of omission rather than saying something outright not true. But I am not sure if that's RAI. "You try to fool someone with an untruth" is kind of vague. In my experience, whether I say something that is factually correct matters less than if I am saying something I earnestly mean as far as how well I can avoid getting nervous or what have you. But I also wouldn't make a character roll a second time if they are just following up on or expanding on a Lie they already rolled for.
I take a page from Sense Motive-- "You typically can’t try to Sense the Motive of the same creature again until the situation changes significantly." Successfully Lying isn't about whether you can rules lawyer your words as technically correct, it is about whether you can keep your cool and avoid changes in your body language and what not, which is something that seems better abstracted to a single roll for a social situation.
There's also a question of how it intersects with the Impersonate action. My initial read on Impersonate was that you rolled when you assemble the disguise and compare that to every NPC who sees you moving forward. So if you Impersonate a generic soldier to get inside a barrack, you would roll once and compare that to the Perception DC of any other soldiers you meet, and might need to do a separate Lie check to get them to believe whatever story you use to let you pass.
On closer inspection, Impersonate calls for you to roll against every creature you directly interact with. Which isn't unreasonable if you consider Impersonate primarily a matter of the act and not the Disguise, but does mean having a separate Lie check seems unnecessary. Getting an NPC to do something they wouldn't normally do might call for another social check, but that could be Lie or Request or Coerce.
Comparing the PF1/PF2 versions of Discern Lies is also an interesting bit of data. PF2 Discern Lies just gives you a bonus when someone Lies to you. PF1 didn't really reference the general skill rules: "You know if the target deliberately and knowingly speaks a lie by discerning disturbances in its aura caused by lying. The spell does not reveal the truth, uncover unintentional inaccuracies, or necessarily reveal evasions. "
How do y'all normally handle PCs and NPCs trying to pull extended ruses?
IMO lies of omission involve Deception, at least in the sense that a perceptive observer should have an opportunity to pierce the veil.
And to rule otherwise would lead to constant lies of omission (by all parties involved) simply to circumnavigate rolling. And yes, I agree to it being about body language (et al), not just phrasing.
Yes, one roll should cover the larger conversation surrounding a lie otherwise people could simply ask "Say that again?" several times to see through most "at level" lies (again by NPCs too).
I'm not sure you have to roll Impersonate unless you interact with them or give them a reason to look closely (which in the latter case would be them rolling vs. your DC). (You do note that later.)
IMO the Lie check would be more specific than identity which I think the Impersonate check would cover.
I don't think a lie would get somebody to do something they normally wouldn't, but rather what they normally would (albeit based on the false premises you've planted). So that would take a bit of knowledge. I've had players lie successfully, but the target didn't care or was too fearful to do anything about it, etc.
A lie could also set up a solid foundation for a Coerce or Persuade, like pretending to be a commander.
Extended ruses would work the same, except there is the issue of PCs (and players) defaulting to paranoia (except inexplicably when a new PC gets introduced!). Each new conversation could possibly require a new roll depending on its topic. Like if you start bad mouthing the spy's home country, but not if you ask them what meal they'd prefer.
I might start with more rolls, then as the ruse becomes the accepted truth extend the time frame per roll to days, weeks, or even months though I doubt that'd arise in normal gameplay.
Forgot about disguise which might need to be redone every day.
Or if they had to maintain an accent, etc.
Hmm...complicated. Would have to go case by case.