Capturing all the enemy grunts. Killing their leader. Getting a Rod of Rulership. Flying to the moon. Making Zeus my shoeshine boy.
If you said any of that to me in the hypothetical circumstances, I'd have to say I have no idea what you're talking about, and that I was unable to proceed. In fact, I'm pretty sure that's the case now. I don't think I'm understanding what you're attempting to express with these phrases at all.
Details? We don' need no stinking dee-tails!
Not all the time, no. Sometimes it's fine to work without them.
I'm quite comfortable with a player saying they're going to try to bluff the rival, or intimidate them, without requiring them to specify their words. If they do, more power to them, it might get them a bonus to the roll, but I won't absolutely refuse to let them try if they all they've given me is a general course of action, rather than specifying exactly what they're doing.
Apparently there's some objection to that though.
Let's try another example. Imagine a player wants to bash down a door. Do I need to ask how they're doing it? No. If they say they're going to get one of the pillars from the room and use it as a battering ram, well, I can just give them a bonus to the roll. I don't have to ask them to detail it completely. Of course, if it's a magic door with a riddle on it, I can let them solve it, or I can accept the results of an insight roll and a knowledge check. And if it's something even more special, I could have Perception, Knowledge, Arcana, and Thievery checks involved.
I see the situation with the rival as a similar case. I certainly can set things up in advance to be more complex, but if I do, it'll be intentional, and even then, I can have a lot of flexibility as to the exactness of the PC's actions. Not every conflict has to be a real challenge.