Teenage boys in books can often be an attraction. The way in which your imagination allows you to create your own perfect person from the description given is convenient, you can filter out all the unattractive details and focus on those wholly more appealing!
Once a character from a book is immortalised in film or television, we have no choice but to conform to this choice already made for us. My own experience of this being Alex Rider, the super-teenager in Anthony Horowitz's books Stormbreaker (and its disciples).
Horowitz is a talented writer, and knows what and what not to include when concocting a character that appeals to everyone. Alex Rider seems too good to be true, adored by girls and envied by boys.  
Although Alex Pettyfer makes for a pretty good-looking Alex Rider in the film adaptation of Stormbreaker, it somewhat shattered my own mirage and has tainted the way I read forever.
Now, reading new books, I constantly assume that there was, is or will be a visual interpretation of the story and, being the cynic I am, leaves me not bothering to imagine the characters’ looks but instead rely on someone else to do it for me.