Depends how the contract is written between the production company /film maker and the persons they filming.
If its an indie filmaker is doing the series, they might not have alot of money to incentivise the person to be on camera but the person might do it just for the fame, depends on what they agree on before production starts.
Usually the production company has most of the rights or ownership when it comes to producing a docuseries and so the royalities go to the production co.
So when the subject of the dociseries is approached by production co, as person of interest "star" of show, if they negotiate for a percentage of the profit from royalties and it goes into the contract, then they will get it.
So it all depends on the contract.. Everyone on a production set is covered by a contract, even extras, who usually get a daily rate but dont get entitled to royalties etc because its not in their contract.
In conclusion the production house will decide how to proceed regarding remunerating the person, they may pay them a "once off" lump sum to incentivise them to participate or have a more detailed contract. And it goes without saying, if the person refuses to be a part of the dociseries then they dont appear on camera, and the production could replace them with an actor of their choice in a dramatization.
