Okay, then try these steps suggested by another user in a thread on gog.com (http://www.gog.com/forum/the_witcher/hair_twitching_when_conversing_with_characters_abi gail_the_reverend_no_multiple_gpu/?search=hair), so credits go to him ;)
I have a GeForce GTX 770, and experienced the same problem, with hair twitching like crazy.
I fixed it by going into Nvidia Control Panel (Control Panel -> Hardware) and changing some stuff in Manage 3D Settings. The things I adjusted:
1) Set anisotropic filtering and antialising to application controlled
2) Multi-display acceleration- single display performance
3) Power management- prefer maximum performance
4) Vsync (this probably fixed it but I'm too lazy to check, sorry)- Adaptive or On
Hope this helps.
tiens ça marche pour moi