This game is silly, first I cant install it, then I walk like lighting, and need to click 10 time to be able to open an intem, BUt I was able to change the resolusion to "1240-1240"
( 1k something atleast ).
And I keept playing all night.
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Now the next day then I fought I make a new character, ( first one was a pure fighter, this one has stealth 51 at lvl 2 ).
But Now I walk slowly, too slowly ( wel right speed for the game but I now cant run after enemys fleeing ), and I still have the same resolusion, but now the screen is running something like 600x800, everything is BIG :(
AND now the Im a more stealth character I realize I CANT STEATLTH ???
I press "S" and right shift but nothing happen !?!?

It was atleast easier to play the I run light lighting but still could Stealth, but the problem is that I did not do anything, but change the controlls in game, and then I restet then nothing restets ???
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If thisg has chaget tomorrow again this is traly wierd here ?!?

Sounds like you are encountering some bugs. Is your version of Arx Fatalis the latest v1.21? There was a patch released by the original developers recently that improves performance on modern operating systems. GOG has updated the Arx Fatalis installer to v1.21 so if you don't have the latest version you can re-download the game from GOG to get the update.

What OS are you using? What graphics card do you have? What are your system specs? I think a few people have had similar issues with the game, so check some of the other threads on the forum too.

The resolution doesn't change, only interface overlay does.
It was explained in this thread.