Back in March we told you about a rad Witcher 3 first person mod that might never see the light of day. Well, it has as of recently, and some new footage has come out as well that shows it's in pretty great shape.
The downside is it doesn't work in all situations (the game goes into the normal third person once you draw your sword, ride a horse, or talk to someone), so for now at least, you can't get the full-on first person RPG experience.
If you still want to try it, download it here. Once that's set, use the hookfp command in the console to initialize it, and press P to go in and out of first person. You can also use the setfpfov(value) command to change your field of view (extreme values may cause crashes), and the fpzcorrect(true/false or 1/0) command to enable or disable head bobbing.
