Hi Daza - due to the nature and intent of the engine (the maps and movement being grid-based), the player is required to walk along the pre-defined grid as opposed to freely walking around the map (similar to games like Legend of Grimrock as opposed to Minecraft, since the first-person view is geared more towards old-school dungeon crawlers vs. a first-person shooter). However, to help some with the stiffness, you can disable "Limit Mouse Look" in your Game Configuration under the First-Person section. This will allow you to freely rotate the view around with the right-mouse button. Moving forward/backward and strafing will still move the player along the tile grid, but it gives a feeling of a bit more freedom.
I am not completely ruling out some sort of free movement in the long term, but for the time being I'd like to stay with the original intent of it being a grid-based engine. Hopefully I was able to explain that well enough. :)