The Joker says "better make it ten [minutes]", before heading up Gotham Cathedral. Exactly 10 minutes later, the helicopter flies into shot to get him.
Willem Dafoe was briefly considered for the role of the Joker. Writer Sam Hamm recalls, "We thought, 'Well, Willem Dafoe looks just like The Joker.'" Coincidentally, Dafoe would go on to play the archenemy of another superhero, the Green Goblin from Spider-Man.
In his hideout, Joker has a lot of pictures scattered around the room. After he cuts out Vicki's picture and starts waltzing around the room, you can see the same picture that appears around the room of a smiling man with crazy dark hair. The guy in those photos looks a lot like Batman director Tim Burton.
When Bruce Wayne is sitting in the BatCave there is a map of the Axis Chemical neighbourhood and a map of Gotham City. The first map is actually that of Capitol Hill, a neighbourhood in Burnaby, British Columbia (see Hastings Street on the south edge of the map, and Dollarton Hiway on the other side of the water). The other map is of Vancouver International Airport and part of South Vancouver, also in British Columbia.
In the scene when there's the Joker fashion product commercial at the end we see a cartoon Joker head in a spiral circle. That's the same Joker on the cartoon Batman TV show.