How do you stop your AI video from "drifting" away from your original Stable Diffusion composition?

I've been trying to solve a specific problem: keeping an AI-generated video consistant from the first frame to the last. Most of the time, the model starts with my image but then generates random motion that ruins the blocking or changes the subjects identity by the end of the clip. I've found that the multi-frame (First-Last Frame) function is the only way to solve this. Instead of letting the AI guess where to go, I provide both the starting point and the destination.