Double Vision n ""You think maybe you'll trust her, 'cause you've touched her perfect body with your mind."" -- Leonard Cohen, ""Suzanne."" You're drawn to the picture of loveliness on the big screen: the lithe legs, the shapely lines, that elusive blend of grace and flesh -- all this, you sigh, combined with acting talent! Now that's a star! Well, no, sometimes it isn't. Although some top-ranked stars like Michelle Pfieffer and (of course) Sharon Stone and (beyond question) Demi Moore are, more or less, how God made them when they take to the screen, a surprising number of the fetching images we see belong not to the credited performers but to employees of agencies like Body Doubles and Parts, run by Shelly Michelle. Michelle, 29, a former ballet dancer, has subbed in love, leg and swimsuit scenes for such stars as Julia Roberts in ""Pretty Woman"" and Kim Basinger in ""My Stepmother Is an Alien."" Her roster of 300 women and 100 men also stand in for stars' feet, hands, even ears in vital close-ups and commercials.