After receiving some positive feedback and a few questions about how this gif was made, I figured I'd put together a little tutorial. Everything I know about media is something someone took the time to teach me or from reverse-engineering another person's project, so I'm a firm believer in sharing what I know.
Bear with me, since this is the first time I've done something like this and I'm redoing some things from scratch.
First I started by turning my photo sequence into a crispy gif (tutorial here). For this particular project, the frame is relatively static so I checked "Attempt to Automatically Align Source Layers." I renamed all the layers so they would be easier for me to keep track of later in this project.
After cropping, some frames had gaps in the background, so I duplicated some layers and moved them to the bottom to fill in spots (see the bottom edge of the GIF).
Here's where it gets tedious/fun/interesting.
I duplicated all the layers to create foreground cutouts. I grouped the original layers in a folder called 'backgrounds' for organization purposes.
Using the pen tool, I traced Cole's outline and created a vector mask for each of the 10 foreground frames. You can also use the quick selection or your other favorite method of raster masking.
I copied the vector masks onto fill layers (which I set at 80% opacity, normal blending mode). On the foreground photo layers, I also feathered the masks by a few pixels.
I toggled visibility for each frame to show the current foreground photo and all previous fill cutouts.
And that's about it! The final gif was exported using Photoshop's default settings (File>Export>Save for Web (Legacy)).
Pro-tip: if you want to post to Instagram, export as a video over 3 seconds (File>Export>Render Video).
If your gif is shorter than 3 seconds, highlight all the frames in Timeline and click the button that looks like a sticky note to duplicate the frames until you reach 3 seconds.
Feel free to shoot me a tweet/DM/email if you have any questions and let me know if you want to learn about my process for other images!