At Excel passengers sat in emergency exit rows are given a brief awareness briefing by a crew member. Personally I always make sure they take it in and I ask if they are happy with what i've said.
Crew still have the authority to move any passenger they feel is unsuitable as an ABP. This is stated on the forms for booking the extra leg room, noted in the brochure and explained by call staff. With some unsuitable passengers, we re-seat them for take off/landing but allow them the extra room during the cruise.