Throughout history, people have worn masks to take on other personalities and tell stories. The masks that we think of as "Comedy and Tragedy" are actually descended from larger-than-life masks worn more than two thousand years ago in the Greek theatre to let the audience know what kind of play they were seeing. The original masks also contained modified megaphones in the mouth so the actor's voice was amplified for distant listeners. Masks worn by the "chorus" (a group of actors who told the audience what was happening) became the iconic masks of Comedy and Tragedy of the modern theatre.
Make a base for a mask using a human face (or balloon with a strategically-placed store-bought half-mask). Cover the face with a double-thickness of aluminium foil and press carefully to sculpt all the features. Remove the foil and cut two holes for the nose and a small circle for the mouth. Put the foil base back on the model and apply petroleum jelly or other non-toxic lubricant on the "face." If your model is a balloon and mask, spray the outside "face" of the foil base with cooking spray so the foil doesn't collapse.
Mix a solution of paste for your papier mache. Use one part wallpaper paste (or flour) to three parts water or use two parts white glue to one part water to make the adhesive and tear two-by-five-inch strips of newspaper for the body. Add a few drops of wintergreen oil (available at chemists) to keep your glue fresh. Soak some strips of newspaper in the glue mixture for about 15 minutes.
Apply strips lightly over the base until the basic shape and features of the face show clearly. Avoid the holes cut for the mouth and nose. Don't bother with details---those come after the basic shape is set. Set the mask aside to dry overnight and then follow the first three steps for a second mask.
Once the base is completely dry, construct details by rolling paper and covering with strips and add thickness on cheeks, chin or nose with malleable "clay" made with mache adhesive and newspaper in a blender. When adding thickness to the mask, do so in layers, letting each one dry before proceeding to the next. Make one mask with a broad, laughing grin and the other with a down-turned, open mouth that might be a cry of pain. Let both dry completely before finishing.
After the masks are dry, remove the aluminium foil bases and discard. Cut away the openings inside of the lips, two nostrils at the bottom of each nose and eyes on each mask (make narrow "squinty" cuts on the happy mask and open, droopy cuts on the sad mask) with a sharp matt knife. Cut around each face, following the jaw line up around in front of where the ears should be to the top of the head. Then cut straight across the forehead at the hairline so that the hair of someone wearing it would show over the top of the mask. Cover the entire surface with a thin coat of mache "clay" and let dry. Sand the dried surface to make it smooth and remove imperfections, then paint with one colour for a traditional mask or use decorations of other ethnic traditions for unique comedy and tragedy masks.
You can build masks directly on faces without using aluminium foil if you have a willing --- and patient --- subject. Be sure to smear petroleum jelly on your subject's face and keep paste and paper out of your model's eyes, nose and mouth. Add a few strips of cotton cloth between the base and details of your mask for strength --- use the mache adhesive to integrate it with the paper. Cover masks with a coat of gesso or other base before painting to make a smooth base for finish paint. Paint both sides and use varnish or polyurethane to protect your mask. Plaster cast fabric is easy to work with, makes exceptionally sturdy masks and requires fewer layers (it's also heavier) but you'll need garnet cloth to sand it and must use a base coat so that paint will adhere properly.
Allow your work to dry completely (on a sheet of waxed paper atop a radiator cabinet is a good place) before painting --- wet papier mache can grow mould.