Sometimes demonstrators are present because they choose to be. At other times, people in powerful positions hire demonstrators. If you are ever approached and asked if you would like to make some money as a demonstrator, there are only two ways this plays out.
The Person Paying You Wants You to Rally for Him/Her
The person paying people to demonstrate wants to hire people to rally behind him/her. This may be because he/she is supporting an unfavorable position, and wants others to see something more positive. It could also be that this person wants to drum up more support than what he/she has, or more support on the issue (political or otherwise) that he/she is representing. If and when this happens, and you have no qualms with this person or this person's stance, then making money as a demonstrator is great.
The Person Paying You Wants Negative Attention
There are two ways that negative attention plays out. Either the person hiring the demonstrators wants to defeat the popular choice, person, or issue, or said person wants negative attention in order to make a stirring speech that ultimately changes people's minds. It is a bit of a complicated play.
If the person offering to pay for your appearance as a demonstrator wants to defeat an unpopular issue or person, then padding the crowd with extra people is quite an impressive tactic. It ends up making the crown look much larger than it would have been, and gets more people thinking about how unpopular someone or something is. Is is a psychological maneuver that has proven effective in various situations in the past.
If the person hiring you to be a demonstrator wants you to play the negativity card for the purpose of spring-boarding him/her into the spotlight to make a rousing positive speech, it is still a good tactic. The speech he/she presents has to be truly moving and incredible, and if the speech is that powerful, it will cause people to rethink what they think they know about your "boss." It can turn events, such as an election, on its ear when one candidate meets with intense negativity via a large demonstration crowd and comes out ahead in the polls because of a good speech.
Where You Stand
So, you have to ask yourself, "Where do I stand?" Depending on your own like or dislike of a person or issue, and what this person is asking you to do, you may or may not have a moral or personal quandary on your hands. You could refuse to do it, and refuse the pay because of personal convictions. You could also decide that your personal convictions do not play a role in accepting work as a peaceful demonstrator.
Whether you want to hire demonstrators of your own or you are interested in fighting for a cause, talk with other people in your area.