During the film Donnie surrenders to the 3 temptations which Jim Cunningham mentions: alcohol, drugs and premarital sex. The other film that was being played in the cinema where Donnie and Gretchen watched "Evil Dead" was "The Last Temptation of Christ".
During life line exercise activity when Ms. Farmer pulls the board into class, there is a black duster on the board just above the rectangular shape. When she starts distributing life line exercise cards, the duster shifts its position to right. When Cherita comes to the board, it's back in its original position.
The movie is set in 1988, but license plates, book editions, news photographs and products from 1989-1999 appear throughout. These anachronisms fit in with the movie's themes of time travel and altered realities. (Chapter 4 even mentions how things from the future, such as Mayans having arrow heads, are symbols of future being brought to the past - these anachronisms are on purpose.)
Why does Dr. Thurman react the way she does to Donnie's comment about the sky opening up?
She's concerned that Donnie might be on the verge of a psychotic breakdown.
Is the Philosophy of Time Travel a real book? Are there more pages to the Philosophy of Time Travel?
No. Director Richard Kelly wrote it during the editing stages of the film and was intending to incorporate it into the original version but time and budget constraints prevented this. Although the book Dr. Monnitoff gives to Donnie seems to have many pages, only those that are shown on the website and on the DVD were created by Kelly.The Director's Cut restores the original vision of the pages that Kelly wanted.
How old are Donnie and the other characters supposed to be?
Donnie Darko: 16 (his birth date is 5-28-72 as stated on the website)
Elizabeth Darko: 19
Samantha Darko: 10
Rose Darko: 42
Eddie Darko: 44