How I do it:
1. Player says what they are doing (IC speech or OOC statement)
2. GM decides if outcome is uncertain
3. If certain, GM states result
4. If uncertain, GM states difficulty for check, between 5 (very easy) and 30 (nearly impossible)
5. Player rolls (or succeeds/fails if modifier guarantees success/failure)
So if it is impossible, then no roll.
As far as player characters go, I won't go beyond "he seems very persuasive" or "you believe what he's saying is true"; player decides how they react. Partial exception in rare case where we need to separate player & PC knowledge.
Edit: In practice might well allow a check to seduce the high priestess or persuade the guard to look the other way; it would depend on a variety of factors including the NPC personality, which I might randomly determine by rolling a d6. I would be more likely to set a DC of 25 or 30 than outright ban the attempt, but this might depend on what exactly the PC said, & how it interacted with the motivations of the NPC. Eg if the priestess had no interest in being seduced by the PC (eg she's actually a golem)

then no roll. Or maybe there is no way she'd break her vows & have sex with him, but he might persuade her it was ok to go for a romantic stroll in the rose garden, and steal a kiss.
For the PC who has invested heavily in skills (eg the Rogue-19 IMC) I want them to be able to do impressive stuff, certainly in the DC 25-30 range. Comparable to what you get with combat feats and spells. But there are limits.