'Physical interplay', when referred to just as a concept, shouldn't have an article. Take a look at this answer which I wrote earlier today about the lack of of article with concepts/feelings in general: http://www.italki.com/question/338618. In your sentence, you are talking about physical interplay in general, the same as you are talking about slapstick in general. These are simply concepts, so there is no article. However, if the sentence were to continue, you could say this: Simple yet effective as a comedic routine, slapstick relies primarily on the physical interplay between the actors on the stage. In this case, you would use an article because you are specifying what physical interplay this is.