Creating a Surprise in writing is hard. Creating the unexpected, though, is writings bread and butter.
If you want to show it, once, then apply misdirection to guide the audience into thinking something else is happening. To illustrate my point —- Sarah recoiled from the sight of the gun as Phil pointed it at her heart. Her eyes fluttered and her head went back, as her body collapsed towards the floor. Phil stepped back half a step and dropped his guard. It was just enough for Sarah to catch herself, and push the barrel aside as she rose back up ....
Admittedly, my example is lame and only works if people dontdon't know the character.
Things to think about — people have a hard time reacting to two events at the same time. she can side stepsidestep and spit her chewing gun in his eye, the instinctive reaction is to protect the eyes. Humphrey Bogart did this a lot in is movies as Sam Spade when he took someonessomeone's gun away — except it was a lit cigarette into the face.