An added element to keep in mind with premade plots is that they, depending on the partner, can become exceedingly rail-roading, to borrow a tabletop term. In essence, you are stuck to your plot's progression with little to no time or opportunity to err off it and when you do there are always those sorts who become immediately infuriated that you are not playing out their exact wishes with each action, or go out of their way to "correct" the manner in which you portray or play out a character.

To date, the best experiences I have had in roleplaying were collaborative efforts combined. The initial posting person might have the majority of the work done, but shuffling out the inclusions, ideas or desires of the other participant or members does not bode well for longevity. A starting point and background, as others before me said, is a fair way to meet the desired objective.