In the modern world, it's clear that some very influential people have a small effect on the evolution of a language by popularizing certain linguistic constructions as slang, which eventually evolve into full-blown common usage.
How frequently was this in historical terms? Were languages more likely to evolve as a result of changing social communities and dynamics, or via influential speakers who would then impart those changes on their peers as a result of their charisma and/or authority?
One example here that I can think of clearly is Shakespeare, who coined many new words (at least he was the first to write them down), and many of the phrases and styles of speaking he used in his plays have been adopted into common English.