Successful composites are all about layers and matching perspective. There are mathematical formulas that help you match camera positions when you shoot miniatures but it's hard to shoot a miniature actor so for the example you posted the camera is going to have to be a long way from the actor and start out below him.
Perspective always is based on camera position not focal length. You need to not only match the angle but the camera position. Here's your sample green screen footage:
The camera is at the bottom of the waterfall and a long way from the top so you'll have to put the actor a long way from the camera and above the camera to make the fall look real. If you don't have a stunt coordinator and a budget for a fall the easiest way to get the fall is to fly the actor with a simple rig so he can spin around a bit just a foot or so off the ground and then move the camera in the opposite direction of the fall by approximately matching the angle and distance. I'm thinking starting with the actor nearly upside down shot from a drone 30 or 40 feet away and above the camera by about 50 feet. The drone descends as the actor spins around. A couple of box fans close to the actor to blow their cloths and hair around would help sell the shot.
For the close up shooting keying water spray is going to require you to shoot with very little compression. 10 bit color or better will give you a chance to pull a decent key with water spraying but the lighting will have to be absolutely perfect. This will require a professional camera capable of lossless recording in a 10 bit or better format. With a consumer camera you will be better off shooting the water plates separately against a black background and layering them in. You would spray your actor to make him wet but not worry about picking up the water in the air. You'll put that in later using layers.
To work out the angles and position will require very careful planning. The CGI waterfall you supplied isn't really very convincing. The water droplets are way too big.
To pull off a convincing composite using the shot you posted would require a background layer where the lighting matched the CG footage, the CG waterfall, at least 2 or 3 mist layers to give the shot some depth, the actor falling, and two or three additional water and atmosphere elements to sell the composite. The comp could easily grow to 10 or 15 layers.
I hope this helps. It would be really helpful in your planning to experiment with camera angles and position using a still camera and Photoshop. That's how I plan these kinds of shots.
Thank you very much. Your advice has been most helpful.
Just to get it clear the angle and position of the camera stay the same while I move it in the *opposite* direction of water flow?
Also, do you have any advice regarding getting hold of a rig and operating one..?
Thank you very, very much again!
If you are moving the camera instead of the actor then you have to maintain the same camera angle throughout the shot. It's all geometry. It doesn't take long to figure things out when you start running some tests. It's all camera position and angle.
As for rigging, if you are an amateur film maker with little or no budget then be careful and find folks familiar with climbing gear to help you. Make sure you are covered by an insurance policy if you do anything that might injure anyone on the set. Don't loose your future because you were trying to create something amazing without insurance.
if you are a professional then hire a rigger in your local area. Local theater companies in many small towns have folks that have flown actors on the stage. In larger towns there are almost always professionals you can hire. The rigs are the same for theater and film. You just need to find someone that has experience.
In more than 40 years in the motion picture and television business I've never hurt anyone on my set worse than a sliver or a small bruise because I've always hired people that knew what they were doing and every setup includes a safety briefing.
Thanks for all the really handy advice Rick!
When u talk about wires do you mean something like this?
Although obviously different the use of wires is needed. Also, when you say "spin" are you referring to front wards?
Again, your advice has been invaluable and I thank you very much for the detail and time you've put into answering my questions.
Yes, that's a wire or a flying rig.