Sufficient detail and realism coupled with a sense of fair play.
Using Karmilla's example:
Example: /me shoot's Makkal in the face. > Is bad.
* Karmilla Strife attempts to shoot Makkal in the face. > Leaves Makkal open to respond with /me dives behind cover to avoid being shot.
To me this would be best:
1 > Reaches down to hip begins pulling pistol
2 > Begins a dead sprint toward 1
1 > Has pistol ou of holster, raising it takes aim
2 > Barrels into 1
1 > Pistol goes off near 2's ear
2 > Try's to grab pistol
Obviously that involves the active participation of both players, one and two have a chance every line to 'break' the RP but choose to make it more dramatic, more like a story.
Let the moment linger until the dramatic end is too sweet to just let go.
An example would be our incident in the I-RED lounge, you could have said *Makkal throws Aya out* -no more RP.
Just as easily as I could have *handwave Kameirabeatsup all the guards* The letting it develop, the taking of small steps and allowing your opponent to react is what makes it fair.
It was a fight I was happy to lose because it advanced the plot and the development of both characters.
Obviously this takes a level of maturity but even the most immature and unstable characters *cough mine cough* can have sensible people at the wheel.