Step 1:
Draw an Elliptical Marquee Tool selection roughly around
your subject.
Step 2:
Select>Inverse. ( CMD/CTL+SHIFT+I )
Step 3:
Create a new Levels Adjustment Layer.
Step 4:
Change the Layer Blending Mode to Multiply
Step 5:
Add a Gaussian Blur. Filter>Blur>Gaussian Blur.
Step 6:
Adjust the amount of blur to your liking. I tend to land
around 200 for the best subtle blending. When done, hit OK.
As a variation to really darken the corners, repeat the
steps but draw your ellipse a lot larger.
To record this in an action, you'll need to start recording after you manually draw the Marquee selection starting at step 2. This will give you the most flexibility to center and draw interest to your subject.