As I mentioned in my Cinema 4D R21 review, one of the features is the Mixamo Control Rig, allowing you to easily modify animations you obtain from Adobe's Mixamo.
On this article I will share a way you can use C4D's Mixamo Control Rig to modify those animations. This may not be the only process that can be used for this, but I've found it to be really easy to follow.
First, import your animation file that you got from Mixamo into Cinema 4D (if you are like me, you uploaded your custom character and then downloaded the animation applied to your character, but for this article I will use one of the default models found on Mixamo).
When you do this, Cinema 4D asks you if you want to reassign the takes included in the file (usually Mixamo files include 2 takes, one with the animation and the other one that is pretty much useless). I chose not to perform this, but you can do it if you want. Now, play your animation to make sure everything works correctly.
Now, reset your skins to your bind pose. In this case, the character will revert to the basic T pose with straight legs and arms.
Then, create a Character and set it as a Mixamo Control Rig, as seen in the image below. Also, after you do this, under the Components section you can create the Root, then the Pelvis, then Arms and Legs. One quick tip is that, if you hold the CTRL key on your keyboard when you click the Arms or Legs button, you will create both left and right legs/arms at the same time (I'm not sure what the Mac equivalent would be, since I'm running Cinema 4D on Windows).
If you click the "Adjust" button under Object Properties, it will automatically adjust the newly created rig to your Mixamo character, which is a big time-saver.
Since your character already has skinning, you dont't need to bind the mesh to the skeleton, so you can simply retarget the animation. If you select your Root node (under the Character object you created), click on the Controls button and then Retarget All. Then, drag your skinned models onto the Weight Transfer area, and click Transfer Weights.
After you've done all of this, if you move a controller of your Character rig (say, if you pick the foot controller and move it), the character should deform accordingly, since it will follow the control rig, not the Mixamo rig included in the original file. However, if you press Play to playback the animation, the control rig will actually follow the animation (as if the control rig had been the one used to animate your character from the start).
One simple thing you can do is variations of the animation by simply transforming different body parts (like the spine, legs, etc.). However, if you want to make more modifications, including those that change over time, you will need to do some extra things. First, you need to display all the controls of your control rig by selecting the following options. Then, from the Object Manager you can pick the controller you need to modify and add the keyframes.
One thing to keep in mind is the bone placement on your rig's arms and legs before you upload your character to Mixamo, because that can really ruin your whole IK experience. For example, Mixamo rigs tend to keep their arms and legs completely straight, and this sometimes causes the IK aim to be completely wrong, something that will end up adding more work since you'll have to fix that. For that reason, I always rig my characters with the legs slightly rotated, to "hint" at the software which direction they should bend.
And this is it. I hope you found this article useful.