In some situations, two characters will go head to head. They might be arm-wrestling or knife fighting or hacking a mainframe simultaneously, but the idea is the same. Whenever two people are competing directly for a similar goal, a contest roll may be made.
Contest rolls are simple: each participant rolls the die associated with their appropriate skill or ability, and whoever rolls lower wins. The difference is considered, and the Storyteller narrates the results. If there's a tie, there's a tie. That's all there is to it.
As with success rolls, never make a contest roll unless the Storyteller asks for it. Many times the context of the story makes it pretty clear who's going to win.
Probably the most common time for a contest roll to be called for is when a character is in combat. In such a case, the attacker rolls with his applicable weapon skill and the defender rolls with her agility die, acrobatics skill, appropriate martial arts ability, or whatever else fits the situation.
Note that with combat contest rolls, each side gets input into what happens if they win the roll. For instance, the attacker may state that she's punching her opponent straight on the jaw, while the defender may only be trying to dodge and get out a gun. Alternatively, he could try to disarm his opponent, run away, or anything else he can imagine. It's up to the Storyteller to interpret the results intelligently.
Whatever the case, combat in the Window should be fluid, quick, and exciting. Rolls should be kept to a minimum and everyone, whether Storyteller or actor, should lend their narrative skills to the action. While an actor is hunting for the proper die to make a roll, the Storyteller should be describing another part of the action. The actors should explain what there characters are actually doing, not just "I attack it..." A poorly told combat can be an immense waste of time, while a well told combat can be an extremely exciting part of the story... (though it should never be allowed to become the whole story)
The moonlight was dim, but it was enough for Anna to see the gleam of the scalpel in the doctor's latex-gloved hand.
"It's time for surgery," he hissed, then lunged madly...
Anna twisted to the side. She was extremely agile (D6), but the doctor was determined to have blood, and he could handle a scalpel (D10.)
The storyteller tossed a die for the doctor, a 4. "Anna, you squint through the darkness to see the scalpel slashing for your neck." Anna rolled her agility die: a 3, just lower than the doctor.
Anna frowned. "The swing goes high as I duck and roll to the side... 'Enough,' I say as I get out my gun. 'Taste this, you son of a bitch.'" Anna rolled her die, a 2.
The doctor reacted too slowly, rolling a 7, and the bullet tore into his lung. He coughed violently, then fell twitching to the floor.