The painting that occasions comment even from Ray is "The Last Judgment" by Hieronymous Bosch. Bosch-like symbolism recurs throughout the movie (the dwarf is one example), suggesting that Ray and Ken may indeed encounter their own Last Judgment - or that the waiting period in Bruges is akin to purgatory.
In order to create the feeling of the holiday season, Christmas decorations were kept in some streets of Bruges until the end of March. The town council made an official communication to the people of Bruges explaining the reason why.
Except for the flashback, Ray wears a single outfit throughout the whole movie. While he does remove his jacket and unbutton his shirt, he has no other change of clothes. Ken, on the other hand, has several wardrobe changes.
When the characters visit Groeningemuseum they are shown viewing paintings "Death and the Miser" by Jan Provoost (ca. 1515), "The Flaying of Sisamnes" by Gerard David (1498), "The martyrdom of St George" by Unknown artist (1500-1510), and "The Last Judgment" by Hieronymus Bosch (1482).
Ken and Ray check into the hotel under the names "Cranham & Blakely", presumably after actors Kenneth Cranham and Colin Blakely who played two hit-men in a television adaptation of Harold Pinter's The Dumbwaiter, one of the film's major influences.
Harry's strict moral code gets a further insight in a deleted scene, where a young version is played by actor Matt Smith, best known for his role as the 11th doctor in 'Doctor Who'. In it, Harry discovers his partner holding a dead woman in a club, so he marches into a police station and beheads the man who did it, a dirty detective. Ultimately, film makers decided to cut the scene, as the CGI decapitation looked fake.
The trivia items below may give away important plot points.
The last scene where Ray stumbles on to the set and he sees all the costumes: the imagery is akin to the paintings of the last judgement; the animal heads, animal skulls, peasant looking people. Plus the dwarf dressed as a schoolboy represents his sin right before he is shot (his judgement). However, he attempts to save Harry by telling him that the dwarf is not a child despite the fact that Harry tried to kill him, thus redeeming himself, so he lives. Plus he notes that waiting in Bruges may be hell for him, however it was more than likely Ken's idea of Heaven. The waiting in Bruges before his judgement may represent purgatory.
When Ken receives the phone call from Harry, the opening scene from Orson Welles' 'Touch of Evil' is playing on the TV, possibly one of the most famous scenes made with one continuous shot. From this point on, Ken's phone call with Harry is one continuous shot, in a homage to 'Touch of Evil'.
In a deleted scene, Ray and Ken are lying in their beds discussing Ray's murder of the priest and the little boy. Ray ponders over why he was sent to kill the priest, guessing it was likely pedophilia. Ken counters that Harry was behind a land-buying deal that the priest opposed. These two explanations are coincidentally two different motives that are suggested for the archbishop's murder in Primal Fear (1996).
During Ken's suicide scene, the coins that he drops (to clear the way) before jumping are the same coins he tried to use earlier in the film when trying to gain access to the same tower, but was instead told he had to pay in notes. Therefore if he had been let into the tower earlier using coins instead of notes, he wouldn't have been able to jump at all.
Ken is never seen killing anyone on-screen (except himself) despite his profession as a hitman. Although given a gun to kill Ray on the orders of Harry over a botched hit, Ken fails to complete this task after preventing Ray's suicide attempt.