Hey, those quirky Coen brothers like to use the same actors in all their films don't they! Don't they? Sort of.
Here are all the actors who have appeared in more than one film by the brothers with those nearest the top being the ones who appear in the most. Going left to right we have the films with the most frequent collaborators to the least. The honours of most frequent actor appearances are shared between Frances McDormand, Steve Buscemi and Jon Polito with five. The film with the most regulars in is possibly their most famous picture The Big Lebowski with nine, followed by O Brother Where Art Thou and Barton Fink with eight.
Plotting this I started with the obvious stars but when I included all the actors (which meant all the smaller names) it soon became clear that over half of them only appear in two films so hard to call them that 'frequent'. So there are thirteen actors who appear three or more times over fifteen Coen films. A bit of a troupe but overall their acting picks may not quite be as predictable as is made out.
Data from IMDB and this handy Wikipedia list.