Sentence:  The `` moving '' picture of the train or the wave coming at the audience is , to be sure , more intense than a still picture of the same subject , but the difference is really one of degree ; the cinematic element of time is merely <WSD>used</WSD> to increase the realism of an object which would still be reasonably realistic in a still photo . 
Meaning of the word marked with <WSD> tags:  put into service; make work or employ for a particular purpose or for its inherent or natural purpose