Any tips for compositing liquid materials over live action footage backgrounds - C4D & After Effects
Capture a spherical high-dynamic range image on set from the location where the liquid will appear.  You can use either a mirror ball, 360º camera, or stitch together a panorama from multiple images.  Then, use the image in the luminance channel of a material, drop the material on a sky object, and place the sky object in your Cinema scene.  You'll probably want to make use of the "compositing" tag on your scene elements to help control what appears in your render passes (which is where you'll find the object buffer).  You can set up global illumination to cast light from the sky object if you want, but if all you're looking for is transparency, refraction, and reflection, you don't need to.

You can experiment with the HDRi textures already included with C4D, and practice compositing the rendered image before you go to all the trouble of setting up an actual light probe.  Just search for "HDR" in the content browser and pick something similar to your scene.  Often, you can get away with using these built-in ones.

You'll also want to experiment with the render settings to get something usable in your compositing app.  At minimum, you're going to need an alpha channel of some sort, you can use an object buffer to isolate where your liquid is, but you'll also need transparency, diffuse, specular, reflections, and more, depending on how you go about compositing the image.  Much of the specifics will depend on the input image, the liquid you're trying to simulate, your compositing software, and the desired end result.  Experiment, watch internet tutorials, and ask experts for specific advice whenever you get stuck.  Good luck!