I aspire to make photographs that evoke the sense of looking at a painting. But I don't really have in mind what Merriam-Webster's defines as painterly photography: "marked by an openness of form which is not linear and in which sharp outlines are lacking." That strikes me as a nebulous way of characterizing a gauzy, indistinct image verging on the amorphous. I see nothing wrong with producing a photo having such qualities, as long as some aspect of it is strongly defined (e.g., the color gradations or how the shapes contrast with or intersect or mirror each other).
For me, though, the painterly aspect of a photograph can emerge to some extent—in some cases, to a large degree—from how sharply an image is delineated, in conjunction with shapeliness of form and nuanced gradation of color. Another factor is the way the background sets off or complements the figure(s) and chromatic character of the foreground. In my view, if the background is sufficiently "painterly" in the Merriam-Webster sense, it serves as a kind of matrix that lifts the foreground into a realm of its own, separate from the usual context of time and space. And in that quasi non-real environment, the elements of line, form, and hue can be more clearly perceived—and aesthetically enjoyed.