How do you get your AI UGC reaction hooks to look natural? Can't figure it out.

As question states, I've been trying seedance 2.5 and kling v3 but without great success.

My goal is to generate front camera recordings of a person reacting to something (gasp / covers their mouth with hand / shook facial expression) while looking natural. That clip would then cut to a recording of my app demo - something you could call a staple format in the industry at this point, everyone knows this, yet it still works.

I could keep poking the models with different prompts, but this process can get quite pricey real fast. And I am aware of multiple services that solve this, however I'm looking to figure this out myself - otherwise at scale I would be overpaying for those other services again.

Does anyone have any tips or tricks? More in regards to model choice and prompting + references.