It is not a pleasant situation for both, the passenger and the large passenger. It is totally uncomfortable for the passenger and totally embarrassing for the large guy. In addition to that, it is uncomfortable and embarrassing for the staff. Plus, there is no one single rule that applies here, each airline has its own policy regarding this.

##What should you do while you are on ground##
If the large person's body is invading your space and you have noticed this during the boarding, make sure to speak to one of the cabin crew members immediately, make it their problem. You are paying for a seat and it is your right to get a full seat, not a part of it. 

Remember, **the only time you can report this case is on ground**, it is the perfect time for crew or ground staff to find another seat either for you or for the large person, or to find another solution in case no seats. Once doors are closed and plane moved without you talking about it, it means you agree to this situation and no one can do much about it in most cases.

##What will the staff do##
This depends on the airline, some airlines make large people pay, some others upgrade large people to business or first class for free as a part of their services (airline I work for do this if no two seats available in guest class), you could get lucky and get moved to a higher class and leave your seat for the other passenger. Either way, your problem should be solved before plane takes off. 

#Things to keep in mind##
This is an odd and embarrassing situation, depending on the airline the sized passenger can be deplaned from the airplane or asked to pay for an extra seat, you also can be asked if you like to wait for another flight! everything is possible if no extra seats are available.