Chemistry. It's a word tossed around in plenty of movie reviews, generally to diagnose whether two actors have it or not. Chemistry can be in the eye of the beholder: some critics may disagree, but if the chemistry is really there, it will show in the way the film catches on. No one can deny that Billy Crystal and Meg Ryan had it in When Harry Met Sally, or that William Powell and Myrna Loy, or Woody Allen and Diane Keaton had it in their many films. But for every hit, there are many, many failed experiments. Here are seven of the most (or least) memorable.

1. Fred Astaire and Joan Fontaine in A Damsel in Distress (1937)
For whatever reason, Astaire decided to break up his hit formula with Ginger Rogers and make this movie without her. His new partner? Joan Fontaine, best known for playing mousy, quietly pretty types (Rebecca, Letter from an Unknown Woman, etc.) and definitely not a song-and-dance woman. Poor Joan was taken to the mat for her lack of singing and hoofing, although the film actually isn't that bad. The ultra-witty P.G. Wodehouse adapted his own novel, it won an Oscar for its dance choreography, and it features another great team: George Burns and Gracie Allen.

2. Hugh Grant and Sandra Bullock in Two Weeks Notice (2002)
Every so often some Hollywood executive gets the idea to team up two big stars, hoping that their massive appeal will translate into screen chemistry; it mostly doesn't. These two romantic comedy masters, who have been wonderful in other films with other people, came together like a dull, wet flint, unable to strike even the most meager spark. Another infamous example of this type of casting came in 1986: recent Oscar winners Jack Nicholson and Meryl Streep came together for Mike Nichols' Heartburn. Before it opened, it had lots of buzz. After it opened, it had more of a stench.

categories Cinematical