Baroque paintings, Massacre of the Innocents included, typically featured bright pastel highlights that almost floated off the canvasses contoured in high contrast by dark shades leading often to black & chiaroscuro points within the frame. This technique gives paintings of this period a very tactile visual quality. Some have said even that between the monumental scale of many Baroque paintings and their highly detailed depictions and dramatically contrasted contours that they have a sculptural quality to them. Indeed it must also be noted that there was an explosion of sculpture making during this period that no doubt influenced many of the painters.
The intensity of it's subject matter and the depiction of brute force mid action gives this painting a sense of aggressive motion that would have stirred in viewers the same kind of reactions that people today experience watching horror films in movie theaters. And indeed in much the same way that those films are hyped and anticipated so was this painting.  Rubens was an extremely prolific painter and created works on a grand scale and in order to accomplish this he trained and employed painting assistants. These assistants no doubt, and Rubens himself who was a very social individual, helped to create a buzz among seventeenth century collectors who were hearing persistently circulated rumors of the paintings progress and impressiveness well before it was ever exhibited.