In a story where the villain is a Worthy Opponent, or if the hero and the villain are Not So Different, sometimes it's easy for the hero to predict their antagonist's every move. His Sidekick asks him how he knows what the Big Bad is going to do next, and the hero responds by saying it's because he knows that if their positions were reversed, that's exactly what he would have done.
This is often used to show that the hero and the villain do in fact have more in common than the hero is usually willing to admit, or that they're on the same level intellectually. The line is most often uttered by Anti Heroes or reformed villains who know the tricks of the trade and are able to think like a villain. In Darker and Edgier stories this may push the hero into morally gray territory, letting the audience know that this character who they thought was a good guy may be no better than the villain himself. He doesn't blame the villain for kicking his dog, running over his little sister and devouring that orphanage full of babies, because placed in his antagonist's situation, he would have done the same thing.
In speculative fiction stories, sometimes the hero knows what the villain will do because they may be effectively the same person, in which case it is much easier for the hero to imagine himself in the villain's shoes.
Sometimes this line is uttered by the villain himself, who understands the implications of his actions and realizes that the hero is only doing his job, and tells the hero that he doesn't hold it against him for trying to kill him, and he should just get it over with, either because the villain knows that if he were the hero he would do the same, or because the villain wants the hero to know that he's just that much of a bastard and would be just as likely to kill the hero if he were given the chance. It's also a common line of The Atoner, particularly if he or she is protecting someone from his or her former kind (such as Professional Killers). Of course, kinder and gentler heroes will not take the villain up on his offer in order to show that there is, in fact, a difference between them and that the hero will not stoop to the level of a villain by obliging his request.
Contrast To Know Him, I Must Become Him. See I Know You Know I Know for the results of both sides following this line of thinking to its logical conclusion, factoring the other side's predictions into their own analysis.
- Shishio, the Big Bad of the Kyoto Arc in Rurouni Kenshin, wonders out loud if this is the reason Kenshin figured out his nefarious plot so quickly.
- L and Light of Death Note. All the time. Heck, L says as much in the first episode we actually see his face!
L: After all, Kira is childish and hates to lose.
Soichiro: And how do you know that?
L: Because I am also childish and hate to lose.
- In the Sonic The Hedgehog OVA, Sonic's robot counterpart, the Hyper Metal Sonic was programmed through Sonic's life data, knowing his likes, dislikes, personality and all of his thoughts. Being on the same wavelength, this leads to their even fight.
"You might know everything I'm going to do, but that doesn't matter since I know everything you're going to do! STRAAANGE isn't it?!"
- In the Batman series' event Knightfall, the Batmobile is stolen. Batman gets it back, starts the car...and it explodes. Fortunately, he guesses this just in time to leap out of the car. Robin asks him how he guessed the villain had wired the car to blow. Batman's response: "Because that's what I'd do."
- In The League of Extraordinary Gentlemen, this is how Moriarty knows that Holmes will try to fake his death after their encounter at Reichenbach Falls.
- Superman once formed a desperate alliance with Darkseid against a supervillain. Darkseid realized the villain was going to go for the food stores to starve the populace with this logic.
- In some Omake in Harry Potter and the Methods of Rationality, titled Lord of the Rationality, Frodo thinks:
A memory of awful clarity came over Frodo then, and a flash of black laughter, and the thought came to him that it was just what the Enemy would do. Only the thought came to him so: thus it would amuse me to do, if I meant to rule...
- Terminator 2: Judgment Day
John Connor: I gotta stop by my house. I wanna pick up some stuff.T-800: Negative. The T-1000 would definitely try to reacquire you there.John: You sure?T-800: I would.
- Variation in Fast and Furious. Brian tries to narrow down a list of suspects with the same name to figure out which one is involved with street racing. He has his FBI partner read off a list of the suspects' cars. After hearing about a Nissan 240SX with an illegal modification, he remarks that he's the one. His partner asks how he knows this and he replies "Because that's what I'd drive."
- In The Chronicles of Riddick, Riddick seems to know everything that happened in the fight that he wasn't there to witness, and then the others escaped in the chaos. When asked how he knew all that, he said it was his plan.
- In Saving Private Ryan The Squad comes under attack by a sniper in a French village. The squad's sharpshooter Jackson spots a tall church tower and says, "That's where I'd be".
- In Predators, being a veteran hunter himself, Royce mentioned in detail how the predators would approach in killing their prey, quoted this exact line "that's what I would do".
- Mombasa takes it one step further, giving a detailed description of a sadistic trap for human prey, then saying, "I know because I have done this."
- In Pearl Harbor, one of the naval intelligence analysts tries to convince Admiral Nimitz that the Japanese are going to attack Hawaii because its what he would do.
- In The Longest Day, General Erich Marcks tells his adjutant he's going to win the upcoming wargaming exercise, where he's commanding the OpFor, because he's going to break all the rules about amphibious attack across the Channel by not attacking the Pas de Calais during a stretch of good weather, but instead Normandy in bad weather. Like they're having right now. He then dismisses the idea of actually doing it in real life because no one is that stupid and/or crazy. However, once he finds out the British are dropping decoy parachutists into Normandy, he realizes immediately that the Allies are that crazy and is the first German general officer to understand what's happening.
- In Neil Gaiman's short story A Study in Emerald, a crossover between Sherlock Holmes and H. P. Lovecraft, the detective tells the narrator that he figured out how the murderers got away based on the fact that he would have done the same thing. The twist is that the narrator and the detective are Colonel Moran and Professor Moriarty, and the two murderers are Watson and Holmes.
- Father Brown, of the eponymous stories by G. K. Chesterton, solves all of his mysteries this way. His gimmick is that, as a priest and confessor, he knows a great deal about how crimes are committed and what kind of people commit them.
"You see, I had murdered them all myself.... I had planned out each of the crimes very carefully. I had thought out exactly how a thing like that could be done, and in what style or state of mind a man could really do it. And when I was quite sure that I felt exactly like the murderer myself, of course I knew who he was."
- Sherlock Holmes claims to be using the same method. In The Musgrave Ritual, Holmes matches his wits against an unusually clever criminal (no, not Moriarty). Afterwards, he tells Watson it was one of his easier cases; normally he has to adjust his deduction of what the criminal would do, since most people are significantly less smart than himself, Holmes. But in this case, what the criminal did is exactly what Holmes himself would have done, making it easier for Holmes to follow him!
- In Harry Potter and the Deathly Hallows, Alastor "Mad-Eye" Moody says that, even if Voldemort had believed their deception about taking Harry away on his birthday, rather than earlier to get him safely away while his protection still theoretically works, the guy would have to be crazy not to have an agent keeping watch, citing this trope as his basis.
- The Quiller novel The Ninth Directive has Quiller tasked to plan the assassination of a "Very Important Royal Person" with the intent that this will show him exactly how the real assassin plans to do it.
- Terry Pratchett's Going Postal: Moist von Lipwig explains to his love interest the way the Big Bad's plan is going to work in. As she (ignorant of Lipwig's criminal past) remarks, 'You sound very certain', he replies, 'That's what I'd do, er... if I was that kind of person.'
- In a previous book in the same series, Night Watch Discworld, Sam Vimes watches an unsuccessful attack against a massive barricade that La Résistance have used to block off a large part of the city; it's a rather inefficient approach, he muses, and if he'd been in charge of leading the attack he'd have tried Cutting the Knot by having men go into the cellars of buildings near to the barricade and break through walls until they could come out on the other side... although he'd had his people go through all the buildings near the barricade on their side and board up every cellar door they could find just in case.
Vimes: "... but then I wouldn't be fighting me, would I?"
- In Desmond Bagley's Windfall, security company owner Max Stafford is on a Busman's Holiday to equatorial Africa. Visiting a suspicious agricultural foundation, he remarks to himself at a strategic location that there's probably a camera hidden up there... or there would be, if he were in charge.
- In Wings of Fire, Qibli figures out that Darkstalker is using a spell to make himself a Charm Person because that is what he would do if he had animus magic. It later turns out that Darkstalker, being a mind reader, actually got the idea from Qibli.
- In Heroes, Nathan cites his belief that the government would capture and experiment on anybody with powers as exactly what he would do in the situation. And then he did.
- In season one of Lost, Sawyer, after being stabbed by Sayid, tells Jack that he should just let him die, saying that he knows it's what Jack wants to do and that he would do the same to Jack if he were in his shoes. Of course, Jack saves him anyway.
- Cameron uses this to predict the actions of enemy Terminators in Terminator: The Sarah Connor Chronicles. Possibly a call back to the T2 example mentioned above.
- There's a scene in Babylon 5 where Sheridan predicts a particular move by the Shadows because "that's just what I'd do." Delenn is horrified by the notion of Sheridan thinking like the enemy.
- There's an episode of Criminal Minds that pretty much epitomizes this trope, "Elephant's Memory". Spencer Reid has insight on the case because he identifies with the unsub, as they shared a history of being bullied in school. At one point he uses this exact phrase when asked how he knew where the unsub would be.
- As a white collar criminal hired to catch white collar criminals, Neal Caffrey of White Collar makes good use of this trope. He frequently deduces how criminals must have hidden their tracks by thinking over how he'd do it.
- In the Star Trek episode "Balance of Terror", this is Kirk's comment after the nameless Romulan commander dodges one of the Enterprise's attacks: "He did exactly what I would have done. I won't underestimate him again." The Romulan commander does pretty much the same thing, guessing Kirk's actions and motivations while saying "If I were their commander, that is what I would do."
- Stargate SG-1: This is how the real SG-1 bests the evil, alternate universe team in "Ripple Effect." Of course, they had the added advantage of being basically the same people as their opponents.
- The Doctor Who two-parter "The Rebel Flesh"/"The Almost People" has a lot of this, since all the one-off characters — and the Doctor — get duplicated throughout the course of the story. Most impressively, one of the doppelgangers manages to guess a password that her original created after the split happened, because it's the password she would have used.
- This is how the titular Castle helps solve crimes: by being Genre Savvy and knowing what he would write in a given situation.
- Dexter has an exchange in the same spirit as the trope when it comes to eliminating a suspect:
Dexter: The guy we're looking for wouldn't turn dead dirty things into living cartoons, he'd find that pathetic.Debra: How do you know?Dexter: [Beat] Because it is pathetic.
- Person of Interest. Ex-CIA assassin John Reese does this a lot (likely to troll his Technical Pacifist partner Harold Finch) in response to the Villain of the Week's murder plot.
- The Professionals. In "Not A Very Civil Servant", a Corrupt Corporate Executive arranges for The Dragon to kill off an accountant who knows too much. Later the executive goes through the accountant's files and finds evidence that he had been making photocopies of every document as Betrayal Insurance. When The Dragon asks how he knew to look in the first place, the executive reveals that he's been taking exactly the same precautions against his smarter Dragon betraying him.
- The Punisher (2017). In a flashback to his service in Kandahar, Frank Castle cites this trope when he tries to warn his superiors the next objective is a Taliban trap. They don't listen.
- Star Trek: Voyager. In "Deadlock", a Negative Space Wedgie duplicates Voyager and its crew, but only one can survive. This trope works better than usual because Captain Janeway really does know what her other self will do.
Janeway 1: You're going to self-destruct your ship.
Janeway 2: What makes you say that?
Janeway 1: Because that's what I would do if your Voyager were intact and my Voyager were crippled.
- The Shadow Lords in Werewolf: The Apocalypse, a tribe with a reputation for ambition and conniving, have this as their flavor quote in the core rulebook.
- That is the default mode of playing Chess. Also, most computer chess algorithms involve flipping the perspective in order to guess the opponent's moves.
- At the end of the Lara's Shadow Downloadable Content for Tomb Raider: Underworld, Lara confronts her doppelganger after gaining control of it.
Lara Croft: "What the hell are you?"
Doppelganger: "I am you, with the flaws removed."
Lara: "If you were your own master, what would you do?"
Doppelganger: "What would you do?"
- Based on that Lara gives the doppelganger its freedom. It goes on to overthrow Natla, its former master, and drown her in a pool of poisonous eitr.
- In Dragon Ball Z Abridged, when King Kai is relating Goku's fight with Freeza to Yamcha, Tien, and Chautzu, the latter correctly guesses that a desperate Freeza "Blew himself up, along with his opponent" because that's exactly what he tried against Nappa. King Kai's response is "Yeah, but this time it worked.
- In South Park Season 18 episode 2, "Informative Murder Porn," the boys track down the kid who was teaching their parents Minecraft. The kid comes up with an excuse and the boys leave him alone, for Cartman to realize the excuse would exactly be the sort of thing he'd say if he was caught.
- Generals often predict the each other's plans when the required intelligence isn't available in this manner. This approach can backfire horribly if the predicting general failed to understand his opponent is more ruthless then he is, or when the enemy has access to information the predicting general doesn't know exists.
- Some generals also take this one step further. They knew their enemy will try to predict their actions in this manner, and therefore alter their plan accordingly.
- This can lead to some pretty interesting battles. For instance, during WWII the Germans thought the invasion of Normandy was a diversionary attack while the assault on Pas-de-Calais (which is geographically closer to the British Isles and also was a natural harbor with a well-established port, and therefore the more reasonable target) was the real push; in reality, the situation was reversed. The Allies were perfectly aware the Germans had predicted they would strike Pas-de-Calais, so they encouraged that line of thought with a massive misdirection campaign and struck elsewhere.
- As an example of how circular this can get, the Germans distrusted intelligence that indicated Normandy was the real invasion for some time because the Allies had gone to extreme lengths to plant deceptive intelligence prior to the invasion of Sicily the year before.
- This was taken Up to Eleven in The American Civil War, where the officers on both sides generally all attended West Point, had the same instructors, studied the same books, et cetera. Some of them had even fought together in the Mexican-American War. The two combined tropes led to the Civil War being the U.S. bloodiest war.
- It could also be said that this was the case for Napoleon's greatest victory, Austerlitz. There he out thought his opponent by predicting that they would attack the weak point on the French right wing. Napoleon used it as a trap to lure the Austro-Russian army off of the high ground so his troops could attack and capture it, cutting his enemies army in half in the process.
- A common mistake that gets made as a result of this is known as "mirror-imaging". The assumption that the enemy will use the same tactics you would if the situations were reversed can be deeply flawed if the enemy's cultural background and tactical doctrines are sufficiently different from your own. This is why the United States and Soviet Union were often completely wrong in assessing each other's intentions during the Cold War.
- Some generals also take this one step further. They knew their enemy will try to predict their actions in this manner, and therefore alter their plan accordingly.
- Frank Abagnale of Catch Me If You Can, his autobiographical book and movie. Eventually, he ends up working with the government tracking down conmen like himself. The few scenes you see of him doing this in the movie include him making similar remarks to the trope name.