<Li> Gene Kelly as Don Lockwood . His performance in the song "Singin' in the Rain" is now considered to be iconic . </Li> <Li> Debbie Reynolds as Kathy Selden . Directors Gene Kelly and Stanley Donen insisted that Reynolds was always first in their minds for the role . Although the film revolves around the idea that Kathy has to dub over for Lina's voice, in the scene where Kathy is dubbing a line of Lina's dialogue ("Our love will last' til the stars turn cold"), Jean Hagen's normal voice is used . </Li> <Li> Donald O'Connor as Cosmo Brown, Don's lifelong pal, and vaudeville partner, who becomes the head of Monumental Pictures' music department . </Li> <Li> Jean Hagen as Lina Lamont . Fresh off her role in The Asphalt Jungle, Hagen read for the part for producer Arthur Freed . She did a dead - on impression of Judy Holliday's Billie Dawn character from Born Yesterday, which won her the role . </Li>

Who played cosmo in singin in the rain