In truth, the rigs that come with Unity Assets are almost impossible to use for animating in Blender - quite often the bones are disconnected and each faces upwards/outwards.
After doing some research into motion capture software (particularly using the Microsoft Kinect, but more on that later) we worked out why this might be - but it doesn't help us, when we want to create our own, custom animations from within Blender. We need to throw this set of bones away and use our own rig to animate the mesh.
And to do so isn't exactly trivial. It's not especially difficult either - at least, not to get a mesh moving to a skeleton. But it is tricky to get it working as well as the original Unity Asset. This is mostly because - it turns out - of weight-painting.
So far, whenever we've parented a mesh to a rig, we've used the with automatic weights option. Which makes it work. But not without problems. The most obvious example of this is around the hips; check out this walk animation.
Watch the bit at around 17 seconds in, where the character walks towards the camera. Notice the "trouser line" where the waist meets the hips. That moves around quite a bit. Which is fine for an organic shape (a humanoid shape covered in fur, for example) but if this character was indeed wearing a belt, things would look a little bit funky.
And that's because the hips - in this model - are weighted above the belt-line. Compare that to one of our imported models.
In order to display the weights on each bone, there are a couple of things to do first - obviously we need to be in weight-paint mode. But in this mode, there's no easy way to select a bone on the model - luckily we can use "vertex groups" to select the points on the model that are effected by each bone.
If we move the hip bone on the model above, there's a very definite "line" where the deformation stops acting - just below the "belt-line".
When we inspected the weights on our imported models, we noticed that where the effects of different bones start and end, the weight painting is the same for both bones.
Note how the lower leg bone affects the shin area mostly, and the knee area is green (affected quite a bit, but not any further up the leg). The upper leg bone affects the thigh area and the knee area is also green. The affect of the upper bone goes no further down the leg than the knee. This ensures that there is no "overlap" between the two bones, so they do not "compete" for influence over the mesh.
Which means our next Blender project will be to rig a model, apply some custom UVs, and weight our custom rig to create a similar behaviour. Right now it's getting a bit late, so that will have to wait for a few days....