They may sizzle on the red carpet, but love-crossed stars often stumble when they work together on the big screen.

Madonna crashed and burned 20 years ago when she starred with her first husband, Sean Penn, in "Shanghai Surprise." In 2002, she flopped again, playing the lead in a remake of the Italian classic "Swept Away" directed by her husband Guy Ritchie.

Now Jennifer Lopez, who famously fell flat starring with former fiancé Ben Affleck in "Gigli" and "Jersey Girl," is at it again. She just finished filming "El Cantante" with her third husband, singer Marc Anthony.

But after so many duds, why do celebrities like Madge and J-Lo continue to make movies with their off-screen flames?

"For J-Lo, she probably does want to make money, but for some reason she just keeps making these terrible films," said Felicity Loughrey, the New York contributor for Australian Vogue.

It seems that on-screen magic most often happens between couples while they are falling in love, not after the fact.

When Brad Pitt and Angelina Jolie reportedly fell hard while filming "Mr. and Mrs. Smith," movie fans flocked to see if they could spot the moment when lightning struck.

The same has been true in other films.

"With Elizabeth Taylor and Richard Burton, it was their grand affair in Rome during the making of 'Cleopatra' really helped to save that colossal bore," said James Robert Parish, a historian who is working on a book about Hollywood breakups.

Delaina Dixon, a staff writer at OK! magazine, cited Reese Witherspoon and Ryan Phillippe in "Cruel Intentions" as a prime example of a couple who made a successful team on screen.

"Maybe when you're in love on the set you give your best performances," she said.

It's only when Hollywood sweethearts decide to star together again in a production that they get into box-office trouble.

"If they want to work together, there are not always the same equally good parts for both of them," Parish said. "One may have a good part and the other one may other have just a subordinate part, as so often happened to Joanne Woodward in a Paul Newman vehicle."

However, sometimes the stars are just pawns in the game.

"One of the reasons that a lot of these teams are packaged together is that they start out being at the same agency and they're part of a big overall package," Parish said. "Put together two high-priced personalities and if it semi-works, then it's very conducive to wanting to team them again."

Another reason why off-screen lovers like to star in the same films: working together allows stars to keep tabs on one another so they don't "play around with some other co-star," Parish said.

Indeed, Taylor once had production of a movie moved to Paris so she could be closer to Burton.

Another problem with lovers acting together: sometimes the relationship ends before the film even comes out. Director Kevin Smith cut much of Lopez's part in "Jersey Girl" after her affair with Affleck tanked.

Lopez, however, has proven that she can act. She did so with acclaim in Steven Soderbergh's "Out of Sight" and the romantic comedies "Maid in Manhattan" and "The Wedding Planner."

"When she has a leading man that has a little sexual spark to him, as she did with Matthew McConaughey in 'The Wedding Planner,' she really bounces off him well," said Parish.

And the third time may be the charm for Jenny From the Block. "El Cantante" follows the career of Puerto Rican salsa "bad boy" Hector Lavoe. Anthony, himself a salsa singer, plays the troubled Lavoe, who died in 1993 after a rocky career rife with drug abuse. Lopez is producing as well as starring in the film.

"It's part of their heritage, so I'm sure this movie when it comes out is going to get a lot of critical acclaim," Dixon said. "And that's something that means something to the two of them, because it's part of their history and their heritage, and it's the reason why they wanted to shoot that film together."

The result might not always be pretty, but Dixon thinks off-screen lovers who happen to be actors like to show the world how well they perform as team.

"It just kind of declares your love to people, that we not only love each other but we can work together. That's how strong our relationship is that we can not only survive off the set, but on the set."