Fresh blood will be injected into the musical Chicago when Sharon Millerchip takes over from Caroline O'Connor in the lead role.     Taking the post as murderer Velma Kelly in the show which has run for a year is not the first time Millerchip has stepped into the high-kicking dancing shoes of O'Connor.     "I took over from her as Anita, which is another great musical role, in West Side Story," Millerchip told Thirteen yesterday after her first week of exhausting rehearsals.  "We'll have to start marketing ourselves together."     The veteran stage performer from Beauty and the Beast, Phantom of the Opera and Cats believes the "scene-stealing" role is a "gift", admitting that "there are leading roles and then there are great leading roles".     O'Connor is stepping out of the musical to begin filming on Baz Luhrmann's musical alongside Nicole Kidman and Ewan McGregor.     Millerchip is only slightly daunted by the prospect of filling a role made so well known by another performer, insisting that "the role is written so well that you can have so many different takes on the songs and comedy bits".     And will O'Connor be passing on any tips?     "Last night she said there's a couple of tricks here and there that we might sit down and nut out together."