HPS is distinctive in that it is both historical and philosophical at the same time.
Good history and philosophy of science is not just history of science into which some philosophy of science may enter, or philosophy of science into which some history of science may enter. It is work that is both historical and philosophical at the same time. The founding insight of the modern discipline of HPS is that history and philosophy have a special affinity and one can effectively advance both simultaneously.
What gives HPS its distinctive character is the conviction that the common goal of understanding of science can be pursued by dual, interdependent means. This duality may be localized in a single work. Or it may be distributed across many works and many scholars, with parts locally devoted just to historical or philosophical analysis. Intellectual history, for example, serves this purpose. What unifies this local scholarship into an HPS community is the broader expectation that all the work will ultimately contribute to the common goal.
There is no distinct methodology that is HPS. Doing HPS does not confer a free pass to suspend the standards of one field to advance the other. It must be good history of science and philosophy, in that its claims are based on a solid grounding in appropriate sources and are located in the relevant context. And it must be good philosophy of science, in that it is cognizant of the literature in modern philosophy of science and its claims are, without compromise, articulated simply and clearly and supported by cogent argumentation.