The sweat and tears our bodies naturally produce have a slightly different viscosity than ordinary water. Plain H20 doesn't bead on the skin the same way real sweat and tears do. You could use plain water to create stage sweat, but you'll find that it doesn't look as realistic as natural sweat. An old theater makeup trick is to mix glycerin and water together to produce a thicker fluid that looks more like real sweat. You can also use this mixture to make fake tears and even blood if you add a little red food coloring.
Mix equal parts glycerin and water in a spray bottle.
Shake the bottle thoroughly for a minute or two.
Spray the mixture on the actor's face and body as needed.
Add a few drops of red food coloring to the mixture to make fake blood.
Keep glycerin out of your eyes because it will sting.