First of all, you would have to rig your characters such that the body parts you're interacting with can move and react.
As for how you implement this, you can do it in one of two ways:
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Choice A - Animation Only: This means you animate the interactions in your program of choice (say, Blender 3D) and then export those animations into Unreal. This means your interactions are limited by the number of animations you create.
Pros:
- Easy to implement in code
- The fact you're just playing animations means less bugs and unexpected visual issues
Cons:
- Animations take a lot of time to make
- Interaction complexity/realism limited by the quality and quantity of animations
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Choice B - Animation + Simulation: The simulation route means you animate the interactive parts of the body using Inverse Kinematics (IK) and wiggle bones. IK is what developers use to make character's feet match the ground, for example. You would still have to make some animations, but those animations would then be modified by the IK system.
Keep in mind that setting up IK in your animation program and making IK work in Unreal are two different things entirely. Your IK rig from your animation program will not work in Unreal out of the box. You would have to learn how to set it up in Unreal to make it work.
Pros:
- Complex and realistic interactions are possible
- Slightly less animation time since you're simulating parts of it
Cons:
- WAAAAAAAAAAAAAAAAAAAAAAAY more complicated to code
- Requires extensive testing and tweaking since simulations mean the possibility of odd or buggy interactions
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I recommend you only use animations and avoid simulation unless you know what you're doing. If your programmer is experienced then the simulation route might be more rewarding. If you've never done anything like this before then the simulation route will be a tremendous challenge (trust me, I got it to work in the end but it killed me. I am writing this from the afterlife).