West Side Story (1961 film cast)

Differences between the movie and stage versions
1) In the film, the Overture includes the song "Maria," changed from "Somewhere" in the stage version.
2) "Prologue" was extended.
3) "Gee, Officer Krupke" and "Cool" switched positions for the film version. "Krupke" was considered incompatible to the mood created by "The Rumble" and, together with the upbeat "I Feel Pretty", was moved up ahead of "The Rumble".
4) "Cool" was placed in the slot following "The Rumble".
5) On stage, "America" is sung just by Anita (Rita Moreno) and the Shark girls; on film, new lyrics were added which were performed by the Shark boys.
6) On stage, "Maria" and "Tonight" are consecutive, followed by "America", while in the film, "America" comes between the two love songs.
7) "Dance at the Gym" was extended in the film, mostly to accommodate the acrobatics of Riff (Russ Tamblyn).
8) The duet "A Boy Like That / I Have a Love" was shortened for the film, and "One Hand, One Heart" is slightly shorter as well.
9) The stage version includes a full-cast ballet sequence, "Somewhere," which was eliminated in the film. Some of the action of the final sequence - the members of both gangs walking away from Tony's body - repeat similar action from the ballet, and the same music is used.

The changes brought about the introduction of a new character: Ice (played by Tucker Smith), who performs the track "Cool", was created to provide a solid successor to Riff for the later part of the film. This character does not appear in the stage version.

source: http://www.westsidestory.com/archives_faq.php