What primary function does crepe hair serve in theatrical make-up?

Crepe hair primarily serves to mimic facial hair in theatrical make-up. It is a type of synthetic hair often used by actors to create the appearance of beards, mustaches, or sideburns, allowing for character transformation that looks more realistic on stage. The texture and appearance of crepe hair can effectively replicate the look of natural hair, making it ideal for adding authenticity to characters that require facial hair.

The other options do not align with the main use of crepe hair. Lengthening hair or creating sleek hairstyles involve different techniques and products, such as wig styling or hair extensions. While crepe hair might add volume in specific contexts, its primary role in makeup design focuses on replicating facial hair features rather than enhancing head hair.