Beryl: Paranormal? Is that what they're calling your kind these days?
Otho: Don't mind her. She's still upset because somebody dropped a house on her sister.
Barbara: We're very unhappy.
Juno: What did you expect? You're dead.
Beetlejuice: I'm the ghost with the most, babe.
[Beetlejuice angrily kicks a model tree and it falls over.]
Beetlejuice: Nice fucking model!
Adam: This book reads like stereo instructions. Listen to this. Geographical and temporal perimeters. Functional perimeters vary from manifestation to manifestation.
Bernard: Delia, you are a flake. You have always been a flake. If you insist on frightening people, do it with your sculpture.
Beetlejuice: It's showtime.
Adam Maitland: What are your qualifications?
Beetlejuice: Ah, well, I attended Juliard, I graduated from Harvard Business School, I travel quite extensively, I lived through the Black Plague and I had a pretty good time during that, I've seen The Exorcist about 167 times and it keeps getting funnier every single time I see it, not to mention the fact that you're talking to a dead guy! NOW WHAT DO YOU THINK?
Lydia: Maybe you can relax in a haunted house, but I can't.
Beetlejuice: Alright, let's get down to business! I got a card around here somewhere. Here, here, who do I have to kill? Here, hold that for me, would you?
Barbara Maitland: Whoa! Ah!
Beetlejuice: There you go!
Adam Maitland: You don't have to kill anybody!
Beetlejuice: Ah! Possession! Good! [Beetlejuice's voice comes out of Barbra's mouth] Learn to throw your voice. Fool your friends! Fun at parties!
Answer: They died by drowning. Jane's daughter even tells Lydia when she asks what happened. Although Adam and Barbara should have been completely wet for the whole movie, Tim Burton decided to keep Alec and Geena dry as he felt that keeping them wet the entire time would be an uncomfortable experience for both of them.