Generally fill should be very soft so it doesn't produce its own shadow, and it should be very frontal since it's not meant to be another source. And it also should be weak, you don't want to wash-out the contrast and mood.
Now if you are talking about fill on a close-up, you have more leeway for it to be harder since the shadow from it is less likely to show up, unless that person is right up against a wall.
As for whether it should be from the opposite side of the key, or the same side, or from front, or over or under the lens, etc. -- that's a creative choice. It just depends on what looks better and what is easier to achieve, etc.
For example, you may have a room with hot sunlight hitting the floor and decide that the fill should come from low to simulate the ambient bounce off of the floor.
Sometimes it is interesting to fill from the same side as the key -- in this case, it looks more like you are wrapping the same key around the face further, and it leaves a sliver on the opposite side still going to near black, which can maintain some contrast. You see this a lot in Roger Deakin's work -- he doesn't use fill much, but he wraps his key around by using a row of large soft sources end-to-end on the key side and he varies their intensity so perhaps the soft light that is the flattest to the subject is darker than the one coming from the side, so both eyes are lit but there is still a feeling of a side-lit face and some darkness on the opposite edge.
You also see this technique in Jordan Cronenweth's work, like in "Blade Runner". The room is often side-lit but there is a softer 3/4 frontal light as well on close-ups that is dimmer than the main side light, so both eyes catch the light and 1/4 of the face still falls to black.
Of course, the only danger there is if the actor ends up turning their head away from the key side and there's nothing on the opposite side at all, assuming you don't want their expression to be lost.
Modern cinematography tends to use very minimum to no fill but very soft key lighting.
