I have followed some tutorials on how to fill up an animated container in Houdini, but they all involve creating the animation and geometry inside the program. I have a character mesh (watertight, no holes perfect for simulation) and I imported him as an FBX with stored mocap animation data. It plays fine in the viewport so I know the animation works, I just don't know what to do next. None of the tutorials seem to involve animation caches included in the FBX. I started with the Agent node, and then connected an Unpack node to access points, but from there it's a mystery why it's not working. Does Houdini not like FBX animations? I'm open to suggestions or references to tutorials based on what I am describing. Thanks

It should be the same or a very similar workflow. If I understand you correctly, what I would do is this:

1- Put a timeshift after the fbx animation and hold the animation at first frame.
2- Fill the fbx (which is not moving) with liquids.
3- Whenever the fluid is settled make the fbx animation take over (by keframing the timeshift or using a switch)
4- As collisions use the animated fbx.

Just make sure that your fbx animation is hollow, you might want to use polyextrude to give it some space for the liquid to fill.