Long time ago i was coming from AMS to HUY with KLM the flight was full and i found i was sitting next to a guy who was absolutely paralytic smelled foul and had no idea what he was doing. i made a complaint and said i didn't want to sit next to him, i was told i would have to because the flight was full. I then said i would get off then because i wouldn't sit next to him. The captain came and said i would have to sit next to him, i said I would get off then. At this point the chaps mate who was sitting elsewhere and sober said he would swop seats with him, that was ok for me but not for the chap he was going to sit next to. He was moved to the other seat, had to be helped because he could hardly stand. We took off and about halfway to HUY he stood up and was sick all over the passenger sat in front of him, and then passed out in the aisle. I was surprised that he was even allowed to board and i do believe that i would have been allowed to get off rather than make him get off if his mate hadn't intervened.