In 1954, an unknown Shirley MacLaine understudied Carol Haney in The Pajama Game.     Haney had broken her leg.  MacLaine replace her, a movie producer in the audience "discovered" the young performer and a star was born.     It's the sort of stuff dreams and Broadway musicals are made of: scriptwriters love weaving storylines around understudies who step into the shoes of fallen stars.     Reality isn't nearly as colourful.     Just ask David Sommerville.  As one of four "swings" in The Phantom of the Opera, her understudies the romantic lead, Raoul (played by John Bowles) as well as eight other ensemble roles.     "Some nights we can play more than one part and be on stage virtually the whole time," he says.  Other nights we sit backstage and watch and wait.  But that's the job of a 'swing'.  To be at the theatre, ready to jump into a role if someone gets sick."     Sommerville auditioned for the part of Raoul, but never thought it second best to be a "swing".     "If you go to every audition saying that you'll only accept leads, I think you'll be waiting a long time," he says.     Eighteen-year-old Sarah Bakker understudies two performers who play the role of Christine Daaé -- Ana Marina and Julie Goodwin.     "I've only been on once (as Christine) and I can't describe it in words.  It was so amazing ... hopefully there will be more opportunities in the future," she says.     Marina is the principal Christine, but Goodwin -- as her "alternate" -- plays the part twice weekly.     "I've done nearly 60 shows in Melbourne, which is great," she says.     Marina and Goodwin even share a dressing room -- understudies do not mix it with principals -- but this does not mean they copy each other's performance.     As Goodwin says, "We're different performers.  We stick to the staging and choreography, but we're very different in our own right."     The Phantom of the Opera is an understudy story itself.  Covering for the injured prima donna Carlotta, the heroine Christine wins the heart -- and unwelcome advances -- of the Phantom.     Roy Weissensteiner is one of two understudies for principal Phantom Anthony Warlow (the other is Simon Pryce) and never minds the collective groan that greets announcements he will be playing the lead.     Audience disappointment is generally short-lived, he says.     "Also, I love playing the role so much it's worth doing it in any shape or form."