On stage at the Regent Theatre, rain is pourin', folks are singin' and things are swingin' -- so far.     But Singin' in the Rain is a big, big show trying to squeeze itself into an itty-bitty venue built in the 1920s as a picture palace.  The show's water tanks hold 2500 litres.  Some of its costumes are wide enough to block corridors.  Tight though it is, David Atkins was fixated on having the Regent for his boisterous stage version of Gene Kelly's hit film of 1952.     "This show is set when silent movies were finishing and talkie movies were taking over," says Atkins, slipping through the sliver of wing space.     "This was silent movie house and the first big talkie shown here was The Al Jolson Story.     "We're going to five venues on this tour.  All of the other theatres are contemporary and while they are lovely and the people running them are terrific, they don't have the ambience.  And it looks better here than it has looked anywhere because so much of what is in the theatre lends itself to the scenic elements of the show."     Will it fit?  Can it fit?  In the fortnight before Singin' in the Rain's opening night, carpenters were swiftly hammering together a staircase to fit the Regent's shallow stage and stuffing the original from the show's Sydney season into storage.     As for everything else, well, that's what tech runs are for.  Techs are dress rehearsals that allow the cast and crew to get used to a new venue and for newcomers to learn the mechanics of a show.     For the 35 new crew joining Singin' in the Rain in Melbourne, the learning curve is steep.  "The show runs two hours fortyish and it will take us two days to get through this tech," director Atkins says at his campsite in the Regent's stalls.     "Last night, in the three-hour call, we got half way through the first act.  Hopefully today we will get to the end of the first act."     Slow?  No, normal.  During the countless tweakings, stops and starts of Rain's tech, Atkins is remarkably good-tempered.  He perfected his cool on the enormous task of co-ordinating the Sydney Olympics opening and closing ceremonies, not to mention some 20 other musicals.     "I have been in techs that have gone on for days and days and days," he says.  "Some are legendary.  Sometimes shows have been delayed because of them, or previews have had to be cancelled.     "I did a show called Dynamite where you fly horizontally.  Of that show I cancelled three sold-out previews.  The show finally worked, but ... the show was three boxes, with different colours and different environments, and that complicated the hell out of things."     Rain made itself complicated by its spectacular plumbing.  As well as the aforementioned tank, there is a podium covered in rubber matting on which stars Todd McKenney, Rachael Beck and Wayne Scott Kermond prance while torrents of rain thunder down on them.     The water gathers in a hidden trough, is piped up through heating and filtering systems, then pours down again on the actors, who meanwhile are trying to keep up with Gene Kelly's dance moves.     "Two-thirds of the choreography is the original that Stanley Donen and Gene Kelly created," Atkins says.  "When they filmed it, so that they could do their best work, they killed themselves for 30 seconds, stopped the camera, had a rest, then did the next 30 seconds.     "Our cast can't do that.  They have to do the whole thing straight through, eight times a week." Above: Sheree Da Costa and Todd McKenney.