To Know Him I Must Become Him

Dib: You see, Gaz? To defeat my enemy, first I must study my enemy, then become my enemy, then move in with my enemy, then wear my enemy's clothes, then--

Gaz: You're in my light.

—Invader Zim

The hero decides that the only way to defeat the enemy is to understand how the enemy thinks in the most intimate and experiential way possible, to get inside the enemy's mind either literally or figuratively. The hero may accomplish this with the aid of drugs, Psychic Powers, or some other strategy. May result in a Not So Different realization, or even a personal epiphany (or Epiphany Therapy if done badly).

A variant is where the law-abiding protagonists asks a dangerous criminal with a background much similar to the person on the loose to try to suss out how the criminal mind works.

Examples of To Know Him I Must Become Him include:

Anime and Manga

In Monster Inspector Lunge uses this method all the time, narrating it as he goes: "I am X... What do I do now?" For most of the series, X is Tenma or Johan (whom he assumes is a split personality of Tenma).
In Neon Genesis Evangelion, Shinji and Asuka get their asses kicked by Israfel who is "one mind in two bodies". To defeat him, the two do a synchronisation training where they do absolutely everything in complete unison. At the end, they are even shown sleeping in the exact same pose. Due to the power of Soundtrack Dissonance (they helped their synch by timing their attacks to music), it worked.
Soul Eater: this happens during Maka and Crona's second battle. In order to understand Crona's soul, which is hidden by their insanity, Maka goes insane herself.
Played for laughs at the climax of the Alabasta arc in One Piece. While most of the main characters are hunting down a massive bomb set up by Crocodile, Usopp's big idea is to start doing a comical impression of Crocodile in an attempt to figure out how he thinks. This earns him a Dope Slap.

Comic Books

In V for Vendetta, Inspector Eric Finch, in order to understand V's motives, visits the concentration camp where V was driven insane (and from which he escaped), and takes a massive amount of LSD. The resulting "imprisonment and escape"-themed acid trip not only helps Finch comprehend V's mind and thereby discover his hideout; it also leads Finch to confront his moral ambivalence, guilt and "trapped" feeling regarding his job, and ultimately to the V-like realization that he's his own jailor and can free himself.
A somewhat inadvertent example occurs in Batman: The Man Who Laughs. Bruce Wayne, targeted by the Joker and under police guard, takes a non-lethal dose of the Joker's venom so that Alfred can spirit him away in an ambulance and allow him to change into costume. However, before the antidote Alfred gives him takes effect, Wayne has a brief psychotic episode in which he blames all of Gotham for his parents' death and swears revenge. Upon regaining his wits, Batman suddenly understands the Joker's paranoid thought process and deduces that the villain means to kill the entire city.
In JLA# 11, in order to navigate a holographic maze controlled by the mind of The Joker, the Martian Manhunter reconfigures his brain so he can think like him.
In Bookhunter, when Agent Bay discovers that the thief he's hunting is a dark-haired woman, he announces that he's going to get himself fitted for a black wig. He explains, "To catch a thief, you must think like a thief. You must live, eat, and dress like a thief." This is never really followed up on.

Film

Though not literally becoming a different person, Batman Begins had the brilliant line "To conquer Fear; You must become Fear."
Happens a lot in Star Wars. Any time a Jedi thinks he can conquer the Dark Side by turning to it, it ends in him becoming a Sith Lord. See: Freedon Nadd, Exar Kun, Ulic Qel Droma, Anakin Skywalker (arguable), Luke Skywalker (averted), Jacen Solo (a bit different).
In Face Off, FBI agent Sean Archer has plastic surgery to resemble criminal mastermind Castor Troy, who is conveniently unconscious and in custody, adopts his mannerisms, and gets himself jailed to find out the location of a bomb from Troy's brother. However, Archer soon finds himself Hoist by His Own Petard when Troy awakens, forces the doctors to give him Archer's face, and takes his place at home and work.
In Dark City, Mr. Hand volunteers to inject himself with the memories John should have received, so he can better understand their quarry.

Literature

Red Dragon: This is Will Graham's whole modus operandi—and curse. In fact, he had to spend some time in a mental ward just because that's what having to think as foully as Hannibal Lecter did to him.
The whole reason children, and specifically Ender, are trained the way they are in Ender's Game. Only someone who completely sympathises with and understands the buggers can predict their tactics and defeat them, but anyone who could understand them that well couldn't bring themselves to wipe them out. So you train children up and don't let them know they're already commanding real armies in real war; an adult would see through this. (Also, they have nimble video-game-playing fingers.)
Older Than Feudalism: Sun Tzu said

If you know both yourself and your enemy, you can come out of numerous battles without danger.
If you only know yourself, but not your opponent, you may win or may lose.
If you know neither yourself nor your enemy, you will always endanger yourself.

Interestingly, in something of a subversion, he was being very literal. That passage is about the importance of Military Intelligence.

In Terry Pratchett's Discworld novel Making Money, the antagonist Cosmo Lavish is obsessed with becoming Lord Vetinari. Not with taking over Lord Vetinari's power and position, but with actually becoming the man. It's kind of sad, really.
- What's more tragic is the fact that there's a ward of people with the same delusion. Even in his insanity, Cosmo couldn't be original. Although he did win the eyebrow-raising competition later that week so at least he's good at what he does.
- On a related note, any Anthropomorphic Personification that spends much time observing humans invariably comes to take on human characteristics. Discworld's Death is the primary example. This is also said to be because all Anthropomorphic Personifications are a product of the collective minds of sentient races. It may have been described that way in Thief of Time
G. K. Chesterton's priest-detective character Father Brown claims that this personal insight into the sinful, criminal mind is the main reason for his success.
In Dan Abnett's Gaunt's Ghosts novel Ghostmaker, Inquisitor Lilith had, in her Backstory, lost her mother to eldar. She threw herself into destroying them, and then into understanding them. She ends up sealing a Way for them and being carried back to their world.
Harry Harrison's Stainless Steel Rat induces psychosis in himself to figure out how to catch a psychotic criminal. It works (though luckily he guessed that his psychotic self would want to stay psychotic and so he knocked himself out to allow his recovery).

Live-Action TV

Doctor Who, Curse of Fenric: During WWII, British Commander Millington has a room in his HQ that looks just like Hitler's office, so he can think like he thinks. (Millington is pretty unstable—though not a Nazi.)
The Pretender, a series where the protagonist's thing is being able to think like other people, did an inversion in the episode "Once in a Blue Moon": Jarod has to catch a serial killer, and tries to without Becoming Him, because he's afraid that once he starts thinking like a homicidal psychopath he won't be able to stop.
The X-Files: There was an episode where there's a gruesome series of killings. Turns out Mulder's old mentor in criminal profiling is doing it--after thinking like a certain killer he was trying to catch for twelve years, he really did become him.
This is more or less the premise of Criminal Minds. "I'm the unsub..."
Star Trek: Deep Space Nine: The episode Field of Fire centers around Ezri Dax tracking down a serial killer on the station in this way. Made more literal by the fact that one of her previous hosts was a murderer.
In the Law and Order: Criminal Intent episode "Vacancy", Goren and Eames come to suspect that a method actor who was working on a role as a serial killer may have got so method that he ended up killing someone himself. They're right, although his motive is a bit more complicated than that.

Radio

There was an episode of the radio version of The Mighty Boosh where there was a villain stealing animals from the zoo. Howard wants to do a psychological profile, get inside the fiend's head and work out when he'll next strike... and Bob Fossil and Vince suggest flying around in a helicopter firing flares everywhere.

Video Games

In Resident Evil: Survivor, the first light-gun entry into the series, the protagonist is investigating an Umbrella lab island by impersonating the evil director.
In The Suffering, the toxic ghost of Executioner Hermes Haight reveals that during the months leading up to his death, he became obsessed with knowing what his victims felt before they died in the gas chamber: merely watching them yielded nothing, and listening to their phone conversations just made him feel like they knew something he didn't; even having sex with their corpses didn't satisfy his curiosity, so he concluded his obsession by "tasting the gas".

Western Animation

Robin and his Red-X alter ego in Teen Titans.
In one Darkwing Duck episode, Darkwing tries this to find his Evil Twin Negaduck's hideout. Problem is, he's also suffering from that week's Applied Phlebotinum accident. He actually becomes Negaduck. You can find Negs under the Omnicidal Maniac pages; Gosalyn and Launchpad easily find him by following the trail of destruction. Then again, he did find Negaduck. And then accused him of being a fake.
The Simpsons
- In one episode, Ned has taken Bart, Lisa & Maggie to the river to baptize them. Homer and Marge are searching for them.

Homer: To find Flanders, I just have to think like Flanders! thinking I'm a big four-eyed lame-o, and I wear the same stupid sweater every day and - aloud The Springfield River!

- In another episode, they hire an exterminator to kill a termite infestation. The Sarcastic Middle-aged Man says that to find the bugs he'll have to "think like a bug, act like a bug, become a bug!" He then starts gnawing on the table.

Real Life / Truth in Television

Supposedly, General Montgomery kept a picture of Rommel on his desk and tried to think like him, and this brought about his victory. There's a funny line in The Ipcress File where after the First-Person Smartass narrator is told this story (in reference to thinking like the enemy), he thinks to himself that the extra tanks probably helped.
- Certainly, Montgomery didn't fight much like Rommel. Monty was a master of set piece battles where you lay out your troops, plan out everything, and launch a complicated, finely tuned attack. Rommel could do that, but he was at his best in a freewheeling mobile battle.
- It wasn't so much a case of trying to become like Rommel as it was a case of trying to predict his moves. Monty's decision not to pursue Rommel's forces came about because of how well he had studied the Desert Fox. He knew Rommel's strength was in running battles, and he was not about to play to that strength.
Method Acting.
In the 1920's World Chess Champion Jose Raul Capablanca was unbeatable. In one particular 8 year span, he never suffered a single loss. His style defined coolness and precision at that time. He was challenged by Alexander Alekhin, a man who never won against him. Alekhin's style was fiery and risk loving, or in other words, he was easy prey for Capablanca. In 1926, Capablanca's cool and precise style sat down to meet Alekhin's - cool and precise style! In the months preceding the match, Alekhin completely analyzed and dissected Capablanca's style (such preparation was unheard of during that time). He determined that the only way to win was to out-Capablanca Capablanca. He tightly controlled his aggressive and risky tendencies. He even agreed to easy draws because that was how Capablanca would play that position. It worked. Alekhin became the next World Chess Champion.
The above Batman Begins example was deconstructed by stand-up comedian Reginald D. Hunter, pointing out that it was just a way to do what you want. Point in hand, he uses it as an excuse if someone takes offence to his Josef Fritzl routine.

"To conquer Fritzl; You must BECOME FRITZL!"