From Discworld MUD Wiki
You *can* perform covert actions while faded, but it's probably either a harder skill check or there's checks against more skills or both. Either that or witches just have poorer hiding skills.
Log of a simple test on a chicken in Lancre town:
>fade into the foreground by not here You stand very, very still and slowly fade into the foreground. >plant crown on chicken You plant one Lancre crown on the spring chicken. >steal crown from chicken You steal one Lancre crown from the spring chicken. The spring chicken bounces towards you in a springy manner. >fade into the foreground by not here If you fade into the foreground any more, you're liable to go straight through an' out the other side. >score [...] You aten't here.