tbh, Jux, my experience of g'pigs as a child was similar to yours as all my friends who had them kept them in cages outdoors or in the garage, so whenever they had them out they just used to sit like lumps of fur, occasionally chewing something.
I never meant to buy a g'pig - I had gone to the garden centre for a water butt and we needed fish food so went through the pet part - and there was this lost-looking little piglet that I just couldn't leave behind. So I bought it and took it home with me - and because I lived mostly by myself (BF visited at wkends only), this piglet got a lot of attention every evening. It started off very statue-like but after a few days started to warm up to me and THEN! Wow, the difference. This was the best pig EVER - knew its name, would come running from wherever it was (had the run of the ground floor of the house) when it was called etc.
So rewarding when you put the time in with them; when my 1st died (I am carefully not referring to gender as I thought it was a he until it died when I was told it was a she - just too confusing!) I got 2 very scared rescue pigs and they never really got "humanised" properly - too traumatised, I expect. Plus I had 5 by then so they formed their own piggy colony and didn't need me!!
But I admit, that is why I always suggest having an indoor cage/area for them, so that they can interact on a daily basis with their human family, not just be brought out occasionally into a very foreign environment.
As NDP says - start small. An indoor cage/ play area for them will help as well, then you'll also be able to observe and hear more of their general behaviour as well.
Good luck - they ARE worth it, honest!