improvisation: mostly heard of in theatre and music. If realism is the script then observation and response without rehearsal is improvised art.
approximation: incomplete rendering. For sake of presentation, rendering means using the sharpest tool to communicate a realist idea. The psychological interpretation of an artist's technique is simultaneously a documentation of the artistic process. Since a detailed accounting of how the painting was made would be a portrait of the artist. If believing a perfectly truthful reproduction of a real life thing is boring then whatever is upon the artist's canvas is its approximate interpretation of reality.