We’d like to remind players that the Red Keep and King’s Landing are populous areas, full of NPCs. Remember to use them in your RP. :) If you’re in a public area, it’s quite likely that there’ll be NPCs in the vicinity—guadsmen at the gates, serving women lugging buckets of water, carters bringing in supplies, stablehands tending to horses, other courtiers, knights, etc. If events out of the ordinary take place—events that would be disruptive to the normal day-to-day operation of the castle or city—it’s entirely likely that some NPCs will get involved one way or another.
Two particular sorts of NPCs should especially be kept in mind: gold cloaks and “minders” (for lack of a better word). The gold cloaks are the police force of the city and the guards of the castle—they _will_ act to break up significant violence (weapons being drawn, for example) in many cases, even when the violence is a matter between nobles alone. Players should feel free to @emit such cases where violent or criminal action involving one or more parties get broken up by the City Watch. They’re there to keep the peace, and while they may hesitate when it’s their betters breaking it, they won’t hesitate forever.
The other type, “minders”, are a broader category. They can be house guards or household knights, septons or septas. Their primary purpose is to protect their charge, one way or another. Threatening someone when they’ve got their guards around is not something that can necessarily be done with impunity. Younger characters—minors (especially girls) and unmarried maidens—will very often have a septon or (much more usually, for women) a septa with them as a chaperone, making sure they stay out of trouble (including the social kind). A charge who is liable to put a foot wrong is likely to get reprimanded by a septa or septon who is escorting them. Players should keep this in mind when roleplaying behavior that is, again, out of the norm and/or transgressive—that NPCs, even those who might be in their train, are often not likely to agree with this behavior and may even be in a position to tell them so.
Good thematic examples from the books would be Septa Mordane with Arya and Sansa, and the behavior of the Kingsguard when acting on behalf of Joffrey to various effects.