Throw It In: Both the lines "dat da da, we're dead" and "thank-you for nothing, you useless reptile" were ad-libbed by Jay Baruchel. Other lines were also improvised, including much of Snotlout's dialogue in the scene where Gobber tells the teens how he lost his limbs.
What Could Have Been: Crossing over with Executive Meddling, the film was originally going to be very close to the book, considerably Lighter and Softer than the end result; concept art shows a much younger Hiccup, Snotlout as a girl, a much smaller Toothless, Astrid with huge braids giving her a Helga look, and a lot more gags. Hiccup's mom was going to be in the film and act as a voice of reason. The directors thought she took away from Hiccup and Stoic's relationship so they cut her out.
Although Snotlout was male in the original books, meaning that in that case they averted using Gender Flip - most likely when they decided to create Ruffnut, thus give the group more gender balance without making Snotlout a girl.
Toothless' design was originally meant to be more canine based, but it was changed to be feline based instead after one of the directors saw a screensaver of a black panther.