Mark, I concur: In Bruges is a gem of a movie. That line you referenced — “Interesting things happening to interesting people is basically the only rule of screenwriting” — is courtesy of Brian Duffield and it’s definitely applicable to this movie. The twists and turns of the relationships between the story’s three central characters, especially given the nature of their jobs as hit men, offers a fascinating character study.
It also has that McDonagh ‘thing’ whereby he deftly combines drama, conflict, and violence with humor. A completely engaging movie which lives well within the space of this screenwriting mantra: Simple plot. Complex characters.