The relationship possibilities are myriad: a same-sex relationship, roommates, parent-child, brother-sister, disabled person-and-caregiver...all qualify (under Wisconsin law) as domestic relationships if the people live together.
Step 3: Develop Characters and Set the Scene
Once you have the situation--let's use a mother and daughter--the next task is to develop believable characters. Give them some history and a reason for being at odds. We could have a mother who is angry because her grown daughter is still living at home and showing no signs of wanting to become self-sufficient. Or a domineering mother who demands that her daughter stay home to take care of her, preventing her daughter from having any sort of life on her own. Flesh out the characters and history in a short paragraph or two of background for your role players.
Imagine a trigger event for the current crisis (perhaps the lazy daughter ran up the phone bill or the dutiful daughter took a step to assert her independence), causing the mother to become angry. Write a one-sentence description of the event for your role-players and a short call description for the officer to receive from dispatch. For example, "Respond to 123 Main Street for a disturbance. Neighbor heard shouting."
Decide which character will be the aggressor and how that will be unexpected. In our mother-daughter situation, we could have a significant size difference, or make one of them disabled, or make one of them be a well-known public advocate of non-violence.
Step 4: Write the Script
You can't literally write a script because you can't entirely control the plot: you don't know how the officers will respond. You can, however, direct your role players. The easiest way to proceed is to give them a starting point and then develop several "if-then" plot branches.
For example, you might decide that the victim in your domestic should be extremely distraught and weeping when the officers arrive. The other half should greet the officers at the door by saying, "I'm so glad you're here. You can see she's totally out of control. I'm at my wits' end." Next, you provide your role players with several options, depending on what the officers do. For example, if the officers properly separate the halves of the fight and interview each separately, direct your role players to gradually reveal what actually went on. If the officers fail to separate the parties, you might direct your role-players to continue to verbally spar and even escalate to some pushing and shoving if the officers do not control and separate them. Continue to provide direction for as many variations as you can think of.
Realize that you will never be able to dream up every single thing the officers might do. Someone will always surprise you. However, if you spend the time up front considering all the possibilities that occur to you, you can set the tone so that if your role-players do have to ad-lib an unexpected plot twist, they can do so in a way that is consistent with the direction you have set.
An important part of this step is setting limits for your role-players. For each variation, identify actions that the role-players may and may not take. For example, you might decide that if the officers separate the parties, each party may refuse to give information but may not lie outright. Or if the officers don't separate them, they may engage each other verbally but may not try to do so physically. What the permitted and prohibited behaviors are will of course vary, depending on the purpose of the scenario and the physical surroundings. You probably don't want a knock-down-drag-out fight taking place in a borrowed office!
Step 5: Give the Story a Happy Ending--or Not
The final step is to decide what constitutes acceptable outcomes for the scenario, and conversely, what would be unacceptable. In our domestic, for example, acceptable outcomes would include correctly identifying the predominant aggressor and deciding whether there was probable cause to arrest. Unacceptable outcomes would include failing to control the situation or arresting the wrong person simply because he or she was the "obvious" suspect.