My personal rule is that I would never leave the engine running and not be
at the controls. Now, if I am flying with another pilot, either of us can
get out with the engine running and if PIC is getting out, we have a routine
for passing control of the aircraft that we have briefed. I have pilot
friends that will run up to hand me something or say something and I have no
problem with that, although I am still watching what they are doing and
anticipating a brain fart. If I don't know the person approaching my
aircraft, or know the person but they aren't a pilot,...engine is off. Just
my own personal comfort zone there.

Also, if I'm shutting it down in a hurry, it's with the mag switches not the
mixture.