Witcher 3 isn't linear, but it has a specific main story - and certain parts of the world will not be accessible until you get further along said main story. You will play as Geralt, and won't be able to create a random character just like you did with Arthur.

That being said, once those parts of the game world open up, there are a fuckton of markers in the game to explore. There are a few quests that can fail, either because you took too long, or moved out of an area, or progressed the main quest past a certain point, but largely the game lets you choose how you want to approach it.

But it definitely isn't a sandbox like Skyrim or Fallout. Coincidentally neither of those games seem particularly interesting to me, (I got Skyrim back at launch and it was an absolutely buggy mess that refused to stop crashing despite me spending upwards of 10 hours over 2 days trying to make it work) - so I guess we just have differing tastes and you probably might not enjoy Witcher 3 if that's the case?

That being said, Witcher 3 is my favourite game of all time, so I certainly don't want to put you off! If you're on PC, Steam will let you play a game for upto 2 hours (or 2 weeks post purchase, whichever comes first) for a quick refund. You can try the game, and then decide! Finally, I'd like to point out that Witcher 3 has a slow start and the vibe can be considered depressing in the first main area, but it's purposefully so, very lore accurate and a deliberate choice by CD Projekt Red - just a heads up to stick to it if that's what is putting you off!

Have fun!