Wishful Projection

Prejudices are a funny thing; they can make those who have them delude themselves into believing they know strangers better than the strangers know themselves. Therefore, you sometimes see people in fiction ascribing certain qualities to other people, without having any grounds for doing so. Sometimes, they will even get angry or disappointed if the target does not or cannot live up to their baseless assumptions. If it's a Tsundere that held the image, expect her to hold the guy up to the standards anyway and yell at him if he falls out of line with her perceptions.

Related to Expecting Someone Taller, In Love with Love, Loving a Shadow, and Replacement Goldfish.

This is, of course, very much Truth in Television.

Anime & Manga

 * In Lucky Star, Konata assumes a large foreigner is a violent kidnapper.
 * In the Fullmetal Alchemist manga, Mei Chan hears about the eponymous Fullmetal Alchemist from various people, and forms a mental image of a tall, gentle-manly prince... and is horrified to find the short, rude reality. She later forms a similar image of Al's real body, after Alphonse, offended that she would think him similar to his brother, provides an exaggerated opinion of his own traits.
 * Full Metal Panic! has this happen to Kurtz Webber, who sees Kaname Chidori's picture, and then upon seeing the real thing assumes she must be an angel, until Kaname chews her friend out for a horrible blind date and begins loudly yelling, causing Kurtz to become very disappointed.
 * Played with in regards to Gauron and Sousuke. In the end, it turns out Gauron had been right, at least at that certain point in time, and to a certain extent. Sousuke himself acknowledges it in the novels, when he tells Leonard: "The only difference between him and me, is what is being enjoyed and what is not being enjoyed." Meaning, Gauron enjoyed killing, while Sousuke didn't. In other matters though, they were indeed similar. To the point where Leonard is outright baffled when Sousuke says exactly the same thing Gauron had, in regards to Leonard's plans.
 * Speaking of Leonard, he's definitely projecting a lot on Kaname, including forcing her to witness his traumatic memories, and drawing not-always-correct parallels between their lives. And he gets very pissed off when she finally makes it clear that she doesn't agree.
 * Leo attempts something similar with Tessa, but she knows him too well, apparently.
 * Kannagi has several moments like this where the main character, Kannagi, will often berate Jin for not being her stereotypical vision of a loyal vassal who doesn't think for himself, just as the goddess expects of him. This often leads to powerful arguments between the two.
 * Kirby: Right Back at Ya! has Fumu assume when this legendary Star Warrior Kirby steps out from his containment pod that it will be a tall prince with a slender demeanor and sword, only for it to reveal it is actually a pink puffball that can't even speak. This literally shatters her mental image.
 * In Kurohime, the main character Zero has fallen in love with a powerful witch that saved him as a child, but as he grows older everyone tells him that the woman he fell for was an evil $^#%@, the problem is Zero refuses to listen. When Zero finally does meet Kurohime, she turns out to be a selfish brat who thinks men are toys and will use men to get whatever she wants and take over the world, going so far as even branding men with the kanji for "Dog" on their back..

Live Action Television

 * Mad TV had a sketch called The Average Asian, in which a completely ordinary fellow of stereotypically East Asian phenotype is expected to live up to all kinds of stereotypes.

Web Comics

 * In Megatokyo, a character finds Piro's sketchbook, creates a fantasy about what he must be like, and then gets hostile when he fails to live up to it.