How big do the spikes need to be? I used some conical cardboard shapes to make road cones which worked really well. I painted them with acrylics, but you could use a metallic spray paint with a gloss sealer. They are really cheap and paint up quite well. They do have a seam line where the cardboard meets however depending on the viewing angle you may be able to hide this. Then its just a case of cutting them down incrementally to allow them to "pop" out of the actor face.
For small spikes, with a base diameter of, say, 5mm to 12mm, I shape the end of a bit of pine dowel (or a stick of square pine) on the belt sander, then cut it off and shape the next one. Then I put coat of shellac on the wooden spikes first, to seal the surface. Or PVA glue with a bit of silver powder and black acrylic paint mixed in. Then when that is dry, a spray of silver paint from an aerosol can. Spray enamel sprayed straight onto the bare wood sinks in and goes dull, it needs a few coats. The pva or shellac coats dry quicker, so I get to the final coat sooner.
A crab monster being built at the moment by an associate has spikes made of sharpened 3mm armature wire, so they are already metallic and need no painting. The wire is sharpened on the belt sander, same as the wood. For a polished look you would hand sand it with 3 grades of finer sandpaper, then a bit of Brasso metal polish to finish it off.
If you can shoot in reverse, you could progressively cut the base of the spike off, and stick it back on. If you need to shoot forwards in time, then you need a set of replacement spikes made up in advance so you can replace each one with the next longest one. If there are many spikes, you could have them come out one at a time, or at different rates, so you can re-use some of the same in-between shorter spikes.