You can swap the roles of any two actors in a movie. What do you pick?

Nick Cage and John Travolta in Face Off
- Fucking beat me to it. LoL!!!
- This has been bothering me for like 10 years and it has to come out now even though it’s only kind of relevant to the topic.
  Spoilers for Face/Off if you haven’t seen it go watch it it’s amazing.
  Okay, so Face/Off has the craziest ending to a movie ever. And you’re thinking to yourself “What that the good guy wins in the end and everyone is happy? That’s not unexpected.” But you have to look at the finer details.
  After Sean Archer is back in John Travolta’s body at the end and he’s like “I don’t need this bullet scar anymore, it’s cool, I’ve healed.” He goes back to see his family. He walks in the door and his wife and daughter are like “Yay you’re our dad and not some crazy person again yay!”. The following things that happen need to be broken down individually for it to fully be understood how insane it is what happens:
  • What is with the weird run your hand down the face thing they do? It’s so bizarre and it’s never explained. His daughter is like “Sorry I shot you…” and he just runs his hand down her face like a weirdo. Why.
  • Archer goes “There’s something I have to ask both of you…” and the just brings in some random kid they’ve never seen and is like “he needs a place to live” WHAT?!?! This kid was in a shoot out in a drug den like a week ago. He was raised in a terrible environment. I’m not saying he doesn’t deserve to be placed in a normal family or anything, but the kid needs like all the therapy. He’s going to have emotional problems. Are you really ready to deal with all that Archer? Really? Your family just went through a crazy thing and you’re like “Here is a huge responsibility we need to take on now.”
  • The daughter says “My name is Jamie” and the DOES THE WEIRD FACE HAND THING! Why do they do that? It’s so weird. Is she like assimilating him into their collective. And he, completely unphased by the hand thing in a show of immense self control, is just says “My name is Adam.”
  • Then Archer says “Show Adam to his new room.” And the daughter and Adam run off and then the most insane thing I’ve ever seen in a move happens. Archer looks at his wife and is like “….Okay?” and she goes “nod nod….okay…” and they kiss. End of movie.
  To that last point…WHAT?!?!!?!? Am I taking crazy pills or something? This dude just decides that they’re raising a kid WITHOUT ASKING HIS WIFE FIRST!?!?! If I brought home a dog without discussing it with my wife first, she would be pretty pissed but a WHOLE DAMN KID?!?! A kid, who as mentioned, is going to need intense therapy and extra support? A kid who, it would seem, is there only to replace your other kid who died which is whole ‘nother kind of fucked up (see the life of Salvidor Dali).
  Anyway, thanks for coming to my TedTalk.
  
  Not only that, but it comes off as some weird replacement kid for their dead son. The Pitch Meeting is fun
  
  Wildest TedTalk ever
  
  Now I need to watch the movie again.
- Fucker
  
  Were you thinking of swapping Nick Cage and John Travolta in Welcome to Hollywood?
- Best answer!
- Damn! I commented the same thing before scrolling through the comments....

Let's do a remake of Rush Hour with Chris Tucker affecting a Hong Kong accent and Jackie Chan in black face. Just to see how the world reacts.
- But respectfully.
  
  Indeed. And since Jackie Chan and Chris Tucker became friends, they would probably at least make something fun to watch.
  
  It's all about getting the right shoe polish
- Do you understand the words that are coming out of my mouth.

Andy Serkis and Liv Tyler
- Honestly I think they would both do well in their new roles.
  
  Andy Serkis is basically everyone, so he'd do a better job of playing Liv Tyler than she does.
  Just caught sight of my own face in a mirror - turns out I was Andy Serkis the whole time.
  
  I've thought about it an uncomfortable amount and decided you might be right.
- So now I'm picturing a scene with Gollum kissing Aragorn, and I really wish I wasn't.
  
  You're welcome.
- This; there are no more correct answers to OP's question.

Arnold Schwarzenegger and Danny DeVito in Twins
- They wouldn't have had to act. Danny DeVito is already a perfect specimen of a man.

Brad Pitt as Robert Paulson, Meatloaf as Tyler Durden.

Sigourney Weaver swapped with the various cats playing Jonesy in Alien. Feline badass ruthlessly combats an alien monster with the occasional jump scare by Sigourney.
- Okay this is the one I would legit watch.

Jamie Lee Curtis and Lindsay Lohan in Freaky Friday
- So…just a normal Friday then.
  
  No, I think it implies that they're normally body swapped, and it's weird that they aren't for a day.
  
  It is very much an improvement.

Harrison Ford and Ke Huy Quan (Short Round) in Temple of Dune. I want a little boy as an action hero while Harrison Ford provides occasional support.
- And this misadventure takes place on arrakis?
  
  Interesting. Sub in a Harkkonen for the mad priest with a taste for organ-harvesting cruelty. A nightclub singer in the role of a Bene Gesserit witch who is there for no obvious reason. (Plans within plans?) The Fremen are the little slaves toiling to extract resources/spice.
  In the end, Short Round liberates the oppressed masses and greenery returns to the parched lands. In accordance with prophecy.
  It could work.
- You may be looking for the Goonies. Conveniently, same actor.
  (And I still stand by "pincers of power" although I know it's wrong).
- You want a live action Jonny Quest...
  Why hasn't THAT happened yet?

Nice Cage and John Travolta in Face Off.

Arnold Schwarzenegger and Sylvester Stallone in Last Action Hero.
It's an older movie, but the idea is that a kid gets sucked into an action movie staring Arnold and at one point, he's trying to convince Arnold that world isn't real, so they go to a video rental place to look at the Terminator memorabilia and it's all Stallone.
- Arnold and DeVito in Twins

Barbie and Ken.
- With the exact same wardrobe in every single scene.

Nicholas Cage and John Travolta in Face Off

The male leads of Twins. The plot isn't changed much.

Nick Cage and John Travolta in face/off
- came here to say this myself
  
  As did like 30 other people haha. Didn’t stop me! I’m gonna take their face… OFF!

Last one;
Sir Stewart and Sir McKellen, X-Men

R2D2 and Darth Vader.

Michael Caine and Kermit in A Muppet Christmas Carol.

Christopher Walken and Johnny Depp in Sleepy Hollow
- “Why am I... the... only one who can see that to solve... crimes... we must use our brains, assisted by reason, using up-to-date scientific... techniques?”
  Ichabod Walken

It’s not a film, but David Tennant and Michael Sheen in Good Omens.

Lawrence Fishburn and Hugo Weaving in The Matrix

Carey and Lee Jones, Batman

Ed Norton and Brad Pitt in Fight Club
John Travolta and Nic Cage in FaceOff

Do animals count as actors? Turner & Hooch could be interesting with Tom Hanks drooling and running on all fours and Beasley The Dog playing a detective.

How ‘bout let’s do Leonardo DiCaprio and Matt Damon in The Departed?
- Jack Nicholson and Vera Farmiga is a far more fun swap.

James and Oliver Phelps in the Harry Potter films

The lady and the dude in Titanic. (I don't know actors, sue me.)

Betany and Downey Jr., Iron Man/Avengers.
Jarvis becomes snarky, sarcastic, and inappropriate. Tony is constantly rolling his eyes and removing the "codpiece enchantments" Jarvis keeps putting on all the Marks.

Collin Farrell and any non speaking background actor in any of his movies
- Harsh - but the first of these to make me actually lol

Woody harrelson and Juliet Lewis in natural Born Killers. I want to see her as the pathological badass in him is the unhinged anger-filled character.
- The closest we get to that is probably True Detective S1.

Will Smith and Jaden Smith in “Pursuit of Happyness” at the ages they were at original filming

Christina Ricci and Samuel L Jackson in Black Snake Moan would make for an AWKWARD movie.

The Rock and Kevin Hart in just about anything they co-star in,
Especially if it at any point involves The Rock's character bridal carrying Kevin Hart's character.

Gary Oldman and Tim Roth in Rosencrantz and Guildenstern Are Dead.

Easy, in the movie Twins I'd swap Arnold for Danny.

Kristy Swanson and Luke Perry in Buffy the Vampire Slayer
Ian McKellan and Christopher Lee in The Lord of the Rings
Bruce Willis and Bonnie Bedelia in Die Hard
- Ian McKellan and Christopher Lee in The Lord of the Rings
  I was considering that one.
- Kristy Swanson and Paul Reubens in Buffy the Vampire Slayer.

I'm expanding the parameters here (aka cheating) because they have never been in a movie together.
I would like to see Peter Lorre in any Nicolas Cage role (and vice versa).
- Nicholas Cage in "M". Oh... shit...
  
  Yes. Absolutely yes.

Jaws and Robert shaw

Mr. & Mrs. Smith; Pit as Jolie, etc.

Philip Seymour Hoffman and Gwynneth Paltrow in The Talented My Ripley.
Basically she's the weakest link in that movie so I'd prefer her to have the smaller role.