The character is pasted into the background. The AI model is quite GPU intensive so you don't want to waste render time on parts of the image that aren't supposed to be animated. Even if they remain static like you want, there will often be color drift and artifacts.

For this image, I cropped the character closely and rendered at 544x960. This produces clips 5 to 8 seconds long. I have some color stabilization and rife crossfade scripts to stitch multiple clips together. There was one shot where her hand went out of frame, so I ran a video outpainting pass to fill that in.

I put all the character clips together then ran auto-masking on it. Then a bit of manual touchup on the mask. I went back to the original psd file and made a clean background plate and placed the masked video of the character on top of that.