Some of the key differences I've noticed between portraiture and fashion: 1. Eye contact vs Eyes off-camera. Fashion has a lot of "off camera" glances while portraiture keeps eye-contact because so much of the personality of the individual is captured in the eyes. For portraits I intentionally keep eyes at camera or eyes down her own bodyline.
2. Full length vs Cropped. For me, women have a beauty triangle- the head, shoulders to right below the breasts. This is where I keep my focus in portraiture. Rarely do I include the feet. Fashion, however, completely ignores the individual and focuses on the product (clothing, fashion accessory)
3. Emphasis is on showing off the clothes vs telling a story of the person. Never have wrinkles in the attire or puckers in the pockets created so much drama than at a fashion shoot. Keep those clothes looking perfect or we have to do it all over again! :D