Thanks Moto-P for the side-by side visual comparison.
Let me see if I have this right:
Exterior Differences:
*Headlight guts [same exterior dims and mounting points
*front bumper cover [adds turn signals for FR-S, black plastic mustache on BRZ]
*front fender vent garnish
*emblems [front, rear, name badge, wheel caps]
*Trunk spoiler [optional anyway?]
Interior Differences:
*Dash trim [silver vs. faux CF]
*steering wheel badge
*gauge cluster bezel [all black BRZ, silver/white tach on FR-S]
*Upholstery [black cloth w/red shoulder patches FR-S, cloth/leather all black BRZ, both with red stitching]
*Auto HVAC, push-button start, optional heated seats, audio head-unit