Ever notice those things in movies or TV shows that just pull you right out of it because they are so unrealistic? I'm not talking about other worlds or sci-fi... I'm talking about things based in our reality that just don't fit. Want examples? I've got them.
How about the miracle ultrasound technicians or doctors that can show a pregnant woman a picture of her baby, the heartbeat... even know the sex... without ever actually touching the woman.
Or the woman I saw on a show that had In Vitro, went home and stuck her legs in the air for an hour or two--then took a pregnancy test and it was positive.
How about the people who jump out of airplanes and have entire conversations on the way down. (The air rushing past alone makes this impossible... not to mention the fact that if you open your mouth your cheeks feel like they are going to explode.)
It is for this exact reason that I love an author/screenwriter that I can trust. As soon as we've established the fact that the author isn't going to cut any corners or ask me to make any unnecessary leaps of faith, then I can let myself drown in their story and feel comfortable that they aren't going to suddenly jerk me out for air.
Do your readers a favor... make your characters, faeries, werewolves, aliens, and vampires as believable and real as possible. :) Take the time to pull your reader in and don't let them go. They'll thank you for it later.