Officially:

Self help exit rows must be occupied by at least one person capable of operating the exit. If the row is empty, it will be necessary to move a passenger to achieve this.

If the passenger load is low and the centre section of the cabin has not been allocated due to aircraft trim requirements or the seat has been blocked due to a technical defect, it is not necessary to move passengers to comply with the above.

Passengers who appear apprehensive or not to have understood the briefing should be moved to another seat row.