I want to make a face out of silicone and bond fishing line to particular parts of the face so that I can use the fishing line like a muscle but I can’t find a glue that would work on silicone. Is there another way?
I assume you use a molding silicone or a specialized prosthetic silicone for film and theater masks. The chemical compositions of those kinds of silicone are designed to not stick to anything.
Construction silicone, on the other hand, is designed to stick to as many materials as possible. You could try applying construction silicone to the mask, but I haven't tested if this actually works. There's also the risk that the construction silicone could "melt" through the prosthetic silicone. This melting process usually takes several days to months, so better try it on a waste piece.
A more promising approach is to embed an anchor in the mask where the muscle would be. The simplest anchor would be a key ring or a washer to which the fishing line is already tied. The mask must be thick enough to enclose the anchor on all sides or it might be pulled out. You can achieve a better bond and more natural movements by cuting a piece of mesh (any grid-like material made of metal, plastic or fabric) into the shape of the muscle, tying the fishing line to it and embedding the mesh where the natural muscle would be.
I would look into making holes into the silicone face and feeding the line through.
If you plan on using monofilament line a tiny hole and the line cut at an angle will already work, the line working as its own needle.
Braided line will need stiffening or a needle to feed it through.
Have a good double or tripple knot out of sight to secure the line.