Dueling Stars Movie

So, let's say we've got at least two actors, with about the same level of success and popularity. They might be interested in making similar projects, have starred in similar roles, or may even be buds and want to make a film together; in any case, it is certain that people will love to see these two in a movie together to watch them go at it. Thus, we get a Dueling Stars Movie, a film whose entire reason for being (and the main reason to see it) is to see "those guys" together.

It should be noted that a film starring two bankable actors doesn't automatically qualify as a DSM. Here are a few qualifications that must be met:

1) The stars must be about equal in success and popularity. For instance, Lethal Weapon is not a DSM: people didn't say "Aw man, Mel Gibson and Danny Glover are making a movie together, I gotta see this!"; Gibson provided the star power here (in fact, this is the movie that made Glover a star). Similarly, Rush Hour wouldn't be considered this either: people didn't give a damn about seeing Jackie Chan and Chris Tucker going at it, they wanted to see Jackie Chan kick ass, and Tucker was along for the ride. This wasn't the case with the next two films though, where Jackie Chan was billed under Chris Tucker.

2) We are interested in seeing the film not only for the characters the actors play, but primarily to see them. People didn't go to see Lethal Weapon to watch Gibson and Glover play off each other, they went to see Murtagh and Riggs do that, which is an important distinction. In the same vein, while movies like Harry Potter and X-Men may have big name stars to their credit that we love to see together (Alan Rickman, Jason Isaacs, Kenneth Branagh, etc. for the former; Ian McKellen and Patrick Stewart for the latter), we don't go just to see them, we go to see Snape, Magneto, Professor X, and so on.

Now Face Off, on the other hand, is perhaps the poster boy of this trope. The sheer awesome ridiculousness of the plot was just icing on the cake; what people really wanted to see was "Travolta/Cage" (as the poster proclaimed) to see who could better Chew the Scenery and blow up more stuff. Of course, a DSM need not be merely a action-blockbuster; sometimes we can get films where we actually want to see the stars act: The Lion in Winter is a good example of this, as the main draw is seeing two of the best actors in Hollywood (Peter O'Toole and Katherine Hepburn) give it their all while they verbally eviscerate each other for two hours (Hepburn actually won Best Actress at the Oscars for her performance).

In short, this trope could be thought of as Just Here for Godzilla as applied to the film's stars, and as a result is practically made of And the Fandom Rejoiced. It will very often (nay, almost inevitably) involve copious amounts of Chewing the Scenery, with the stars often making glorious hams of themselves, to the point that the whole movie can be made of pure, juicy meat, especially if the stars in question go after each other in a bout of Ham-to-Ham Combat.

A Sub-Trope of All-Star Cast. Not to be confused with Dueling Movies.

Examples
It was the perfect trap Johnny Depp and Angelina Jolie in TRIP TRAP! [-The Tourist.
 * Face Off: John Travolta vs. Nicolas Cage.
 * The Lion in Winter: Patrick Stewart vs. Glenn Close, in a remake of the above-mentioned O'Toole/Hepburn film.
 * Ocean's Eleven: Nobody cares about watching Danny Ocean and his crew rob a casino; they want to see George Clooney, Matt Damon, Brad Pitt and company rob a casino (and look damn cool while they were at it).
 * The same holds true for the original Ocean's Eleven from The Sixties, except replace Clooney, Damon, Pitt & Co. with the Rat Pack (that's Frank Sinatra's gang—no, not that one, the one with Dean Martin and Sammy Davis, Jr.).
 * Heat was celebrated as the first film to finally star Al Pacino and Robert De Niro onscreen together. Later emulated by Righteous Kill, which was made solely so the two would again star side-by-side.
 * Forget that white kid from Boston (who's supposedly the film's main character), the advertising for The Forbidden Kingdom was all Jet Li and Jackie Chan, which is really the only reason to see it.
 * Hell In The Pacific: Lee Marvin vs. Toshiro Mifune as American and Japanese soldiers stranded on an island together? Oh, hell yes.
 * The Expendables is fueled by this and pure manly awesome.
 * Universal Soldier: Jean-Claude Van Damme vs. Dolph Lundgren.
 * The Towering Inferno with Paul Newman and Steve McQueen, for which "diagonal billing" (one person's name top right, the other's lower left) was invented. In fact, at McQueen's insistance, they were paid exactly the same and had the same number of lines.
 * Moon Child, starring Japanese Visual Kei superstars Gackt and Hyde (of L'Arc~en~Ciel fame). And the fangirls rejoiced.
 * Bowfinger- Let's face it, it's got both Steve Martin and Eddie Murphy.
 * The Bucket List, because who wouldn't want to see two of Hollywood's most beloved actors on-screen together?
 * Tango and Cash: Sylvester Stallone and Kurt Russell team up to kick ass? Yes please.
 * The Prestige: While it's adapted from a novel, its main attraction is seeing Christian Bale and Hugh Jackman screw with each other (no, not that way). Having David Bowie and Andy Serkis certainly didn't hurt, either.
 * Mr. and Mrs. Smith, or Brad Pitt and Angelina Jolie Fight Each Other And Do Sexy Things
 * One of Hollywood's most famous examples is The Sting, where Robert Redford and Paul Newman get into dangerous situations and act manly together, following on from their huge success doing so in Butch Cassidy and the Sundance Kid.
 * The African Queen, which was Humphrey Bogart and Katherine Hepburn's only screen pairing to boot.
 * Catch Me If You Can: Leonardo DiCaprio vs. Tom Hanks.
 * Bob Hope and Bing Crosby in the Road To ... movies—you were either watching to see those two spark off each other, or for Dorothy Lamour.
 * Even though they're only together for one hilarious scene, Limelight was famous for finally putting two of the biggest stars of the silent film era, Charlie Chaplin and Buster Keaton, on the same stage.
 * Baby Mama was basically an excuse to see what happens when you give BFFs Amy Poehler and Tina Fey a film together.
 * Date Night was similar with Tina Fey and Steve Carell as a married couple.
 * And let's be honest, people didn't go to see Mamma Mia! for the plot. They went to see Meryl Streep and Pierce Brosnan sing ABBA songs.
 * The Score is both an acceptable heist flick, and an excuse to get Robert De Niro, Edward Norton and Marlon Brando in the same movie.
 * There were, in fairness, a few good reasons to go see Public Enemies. No-one cared about any of them, except to see Batman square off with Jack Sparrow.
 * "A historical costume drama about Ann Boleyn and her sister fight over the Henry VIII? Booorrrin- Wait, the sisters are played by Natalie Portman and Scarlett Johannson? Well, I guess I'll give The Other Boleyn Girl a shot!"
 * Borsalino: Jean-Paul Belmondo and Alain Delon in the same movie in 1970, meaning possibly at the height of their national (and possibly international for Delon) fame. To find an American gangster movie with two top and highly bankable stars at the time it was made, you would have to wait for... let's say Heat.
 * Kirk Douglas and Burt Lancaster made a couple of movies together including: Gunfight At The OK Corral, Seven Days in May and last one, much later in their career when they were both older Tough Guys.
 * The Way We Were starred Barbara Streisand and Robert Redford when they were both at the height of their careers.
 * City Heat starred Clint Eastwood and Burt Reynolds.
 * The Man Who Would Be King starring Sean Connery and Michael Caine. The movie itself is great but it also inspired these great impressions by Simon Pegg as Caine and Nick Frost as Connery, as seen in this outtake from Shaun of the Dead and this one from Hot Fuzz.
 * True Romance has an all-star cast but the best scene is arguably where Dennis Hopper faces off against Christopher Walken.
 * War was basically an excuse to see Jason Statham and Jet Li duke it out.
 * Mostly because we barely got to see it in The One.
 * It was the perfect trip
 * Audrey Hepburn and Peter O'Toole in How To Steal A Million.
 * Gregory Peck and Sophia Loren in Arabesque.
 * John Wayne and James Stewart in The Man Who Shot Liberty Valance.
 * Fathers Day teamed up Billy Crystal and Robin Williams on the big screen for the first and (let's pray) only time, as the film failed to resonate with critics and the box office.
 * Frank Sinatra and Bing Crosby were the two most popular singers of their day. High Society, a musical remake of The Philadelphia Story, had them singing a duet together—the only time the two of them ever did this on screen.
 * The original Sleuth starring Michael Caine and Laurence Olivier.
 * The Missouri Breaks(1976) starring Jack Nicholson and Marlon Brando.
 * A Few Good Men has Jack Nicholson and Tom Cruise.
 * Mae West and W. C. Fields in My Little Chickadee.
 * Fast Five, the fifth installment of The Fast and the Furious series staring Vin Diesel and Dwayne Johnson. Johnson's character was originally going to be an older guy, but awesomely enough the filmmakers took up a fan's suggestion on Facebook that it would be great to see Diesel and Johnson in a movie together. And it was.
 * A lot of the Frat Pack flicks, but more specifically Tropic Thunder. Ben Stiller, Jack Black, and the recently renewed Robert Downey, Jr..
 * Viva Las Vegas starring Elvis Presley and Ann Margaret. The former who's rock star career cooled a little bit since joining the army and the latter who was white hot after Bye Bye Birdie.
 * Ishtar starring Dustin Hoffman and Warren Beatty is a notoriously unsuccessful example.
 * Charlie Wilson's War and Larry Crowne starring Tom Hanks and Julia Roberts.
 * The Raven, directed by Roger Corman: Vincent Price vs. Boris Karloff vs. Peter Lorre.
 * The Death at a Funeral remake, starring comedians Chris Rock, Martin Lawrence and Tracy Morgan.
 * The Joker and Jack, together in one movie? Sign me up!