Skeptic No Longer

""And I don't want to hear anything about "I don't believe in vampires" because I don't believe in vampires, but I believe in my own two eyes, and what I saw is fucking vampires!""

- Seth Gecko, From Dusk till Dawn

Bob is a dyed-in-the-wool Agent Scully. No matter what Alice tells him, he cannot accept that the truth is anything other than what his own common sense and experience tells him. Usually Alice, the hero, gets about as fed up as she can get with Bob, because Alice knows the truth and Bob. Will. Not. Listen. But then Bob runs into the monster, and suddenly finds that he can no longer pass it off as swamp gas from a weather balloon that was trapped in a thermal pocket and reflected the light from Venus. This shocks Bob into dropping the Idiot Ball very quickly, and very heavily.

In the blink of an eye, he goes from being Agent Scully to being Agent Mulder.

Often Played for Laughs, since the former skeptic is almost always the most rational denier up to this point. Bonus points are given if the skeptic is also an Obstructive Bureaucrat who suddenly comes to his senses. This sort of occurrence also helps to kick the action along since now they can treat it as Serious Business.

Anime and Manga

 * Kyon from Suzumiya Haruhi initially thinks he's the Only Sane Man after Nagato, Mikuru and Itsuki tell him the truth about their nature as well as Haruhi's. Then, Ryoko tries to stab him, traps him in a sealed dimension, turns her arms into energy tentacles, then Nagato arrives, survives getting impaled by twenty spears, chants a couple of spells, and Ryoko dissolves. Needless to say, he's afterwards quick to believe in whatever he sees or hears from these three.
 * Chisame in Mahou Sensei Negima goes from the Only Sane Man into... the only sane man aware that the universe she lives in doesn't make sense at all but whatever. She gets cool age changing pills and magic hacking powers if she plays along, right?

Film
"Stone: Did you see him?
 * The marines in Aliens were pretty much uniformly contemptuous of Ripley's description of the xenomorphs, refusing to believe that they could be as effectively lethal as she was saying. After all, they were rough, tough hardcore marines. Suffice to say they found religion pretty damned quick...
 * Captain Lorenzo in Die Hard 2: Die Harder believes John McClane that Colonel Stuart and Major Grant are working together only after McClane empties a submachine gun (the same kind used by the soldiers in their attack against the terrorist) full of blanks at him. Lorenzo then calls in the cavalry.
 * The page quote comes from From Dusk till Dawn, where the fact that vamprires are real comes as a shock to everyone... but they get over it quickly.
 * In The Sixth Sense, Doctor Malcolm Crowe believes that Cole Sear's problems stem from some sort of schizoaffective disorder... until he encounters something that turns his skepticism on its head.
 * In Split Second, Detective Dick Durkin refuses to believe that the Serial Killer he and Harley Stone (his partner) are tracking isn't actually a human being at all, but rather some kind of monster. And then he runs face-to-face with the thing.

Durkin: That wasnae a him, that was a fucking it!"


 * Seeing the monster also leads to Durbin's line, "We need to get bigger guns!", which under the circumstances was a very rational reaction.
 * Doctor Peter Silberman encountered proof that Sarah Connor wasn't crazy in Terminator 2: Judgment Day. By The Sarah Connor Chronicles, he's living in isolation, away from major cities, raising dogs and keeping a healthy sense of paranoia about killer robots disguised as human beings. He even apologizes for not believing in Terminators, not that it does him much good.
 * However, this isn't the case in movie continuity. He appears in Terminator 3, as Scully as ever, and ascribes his experience with Terminators to psychological trauma.
 * Oh, come on, Indiana Jones. In Raiders of the Lost Ark he remarked that despite being an expert on the occult he doesn't believe in magic, superstition, (and if you liked Indiana Jones and the Kingdom of the Crystal Skull, aliens.) yet in Raiders and Skull he believes it enough to get the hell out of there and or not look at the item that would have killed him.
 * Then again, the item in question was the Arc of the Covenant. Not daring to look at it when some idiot is opening it is just fear of God.

Jokes

 * A skeptical atheist walks around near Loch Ness. Suddenly, the Loch Ness monster erupts from the water and grabs him. In panic, he shouts "Oh Lord, save me!" God answers: "You dare to ask me for help, when you didn't believe in Me one minute ago?" The atheist says: "Have mercy, Lord - one minute ago I didn't believe in the Loch Ness monster either!"

Literature

 * Paksenarrion, from Elizabeth Moon's The Deed of Paksenarrion, gradually changes from a skeptic to a believer to a full-blown Paladin of St. Gird; the Girdish medallion Canna left to Paks when Canna died gives the first signs when it starts warning Paks of dangers.
 * At the end of Victory of Eagles, General Wellesy declares himself a convert to the idea that the Dragons of the Aerial Corps are in fact fully sentient... then declares that he "will be damned if they will become political" and manages to get convicted traitor William Laurence's sentence of death commuted to Transportation to Australia on the condition that he take the most obvious prospective rabble-rouser (Temeraire himself) with him.
 * In Man from Mundania, Grey Murphy refuses to believe in magic after being brought into Xanth, until he encounters a river flowing up a cliff and simply can't come up with a scientific explanation for it.
 * Professor Summerlee in The Lost World.
 * Word of God states that Cloudtail and Mothwing of Warrior Cats stopped being atheists after  in The Last Hope.

Live Action TV

 * In the How I Met Your Mother episode "Matchmaker" Marshall and Lily (both a bit superstitious) are convinced their apartment is infested by a mutant combination of a cockroach and a mouse (which they dub a cockamouse). Robin plays the role of skeptic until the end when she finally sees the cockamouse herself (though we never do), and it's revealed that somehow the cockamouse can fly too.
 * In The X-Files, Dana Scully herself became the believer (with her new partner, Agent Doggett, as the skeptic) when Fox Mulder was Put on a Bus.
 * Lost's Jack after he returned to the Island. Previously he denied everything supernatural or odd, even if it happened right in front of him and other people saw it. Once he became a believer, he went to the opposite extreme-believing everything. Such as lighting dynamite right next to him on a slight chance that he can't die...

Web Comics

 * Partially inverted in this xkcd strip
 * Girl Genius: Carson Von Mekhan was of course initially dubious of Agatha's claim of being the real Heterodyne heir. Seeing Agatha in action wore down his doubts, and they were banished completely when she made the kill-happy Castle Heterodyne back down from killing Herr Diamant for doubting that there even was an heir.

Western Animation

 * Captain Black of Jackie Chan Adventures gets this in the Season One Finale. Shendu? In ma base, steeln d talizmans? Whoa, Nellie! I'm okay now, let's defeat magic with magic!
 * Happens to Eduardo in the first episode of Extreme Ghostbusters. His very first line is "Ghosts. Yeah, right." Then Slimer shows up, and by the end of the episode, Eduardo is a Ghostbuster.