I rarely get to sit at the front emergency exit because usually they’ll charge extra for it. During our recent Fukuoka flight, the plane was half-empty and so me and sis transferred to the front exit after take off. In the middle of the flight, I noticed the FA stop a passenger from going to the toilet, told her to wait a while then suddenly closed off the entrance with a board. Turns out the captain was going to take a leek. Afterwards, she took out the board and the passenger was able to go in.