When creating an RP or an SG you have to think carefully about the plot. Just like when writing any other story, you have to do some world building and have a clear plan in mind for where you want the RP or SG to go.
SGs
For an SG in particular, planning the plot is very similar to writing a story. You have to create an outline of where you want your beginning middle and end to be, because if you don’t go into it with a plan, your SG will be less likely to succeed. However, you also have to plan out where you will need input from the creators of the characters. There has to be places within the plot that are flexible, while still moving the story forward.
RPs
In RPs, there is a lot more flexibility when it comes to planning, because it depends on how plot driven you want your RP to be. Some RPs have their overarching plot of why the characters are there and otherwise being up to the RPers, while other RPs are highly plot driven with certain objectives for the characters to move the plot along, and the rest lie in between.
Once you determine the initial plot of why the characters are there and some basic world building pertaining to the primary area of the RP, from there it depends on how heavily you want the RP to revolve around the plot.
If the plot is extremely important, you have to determine what goal the characters have, and how the characters are supposed to achieve that goal. However, you also have to allow for there to be activities for all of the characters, not just the most active and involved. You have to plot for specific events and how the actions of the characters could change the results for better or worse.
Overall, making certain you know why the characters are there and what the overall goal is, is the most important. The more you have planned with the plot the better you can guide the characters and influence their actions, while still allowing the RPers to put their own influence on the characters and their individual stories.
