A show's longevity depends on a delicate balance between letting things change and keeping them the same. But teenagers grow up and for Glee, set in a high school, the ring of the graduation bell has been ever-present.
The gang from Beverly Hills 90210 famously remained at school until their hair turned grey. The executive producer of Glee, Dante Di Loreto, insists his show was never going to make the same mistake.
''As the show evolved we knew we weren't going to have seven-year seniors in this show. We didn't want to become one of those long-in-the-tooth television shows.''
What is more, he says, the audience is hungry to see the characters step outside the world of William McKinley High School. ''They want to follow characters they've invested three years with into the next phase of their life.''
To that end, the fourth season of Glee is about rebirth, Di Loreto says. ''There are some exceptional new elements that Ryan [Murphy, co-creator] always spelled out from the beginning, that we would introduce new cast, which we have to do in an engaging way, and that we would follow existing cast to new locations and integrate them in a way that is recognisable as Glee.''
One new cast member is the Australian singer-actor Dean Geyer, who came third in the 2006 season of Australian Idol. He will play Brody Weston, a love interest for Rachel Berry (Lea Michele).
''We had a very vigorous and competitive casting process for this season and Dean really rose to the top very quicky,'' Di Loreto says. ''He was charming and had fantastic performance skills. He couldn't be a nicer guy to work with.''
Music is an essential part of Glee's success, with the show's soundtrack albums regularly topping the charts. What Di Loreto finds most striking is the variety of music viewers respond to, from pop hits to Broadway. ''It's fun to watch the audience engage,'' he says. ''When we did an Adele mash-up and released it as a video and suddenly millions of people were sharing it around the world, that was exciting to see. We work really hard to make this show and we care about every single number.''
When Glee launched in 2009 it burnt hot and hard, delivering 20th Century Fox a lucrative sideline in albums, concert tours and merchandise. ''This is going to sound a bit ridiculous but I think we all still feel like outsiders here, like we're getting away with something even getting to have this one on the air,'' Di Loreto says. ''How successful it is is certainly thrilling but most important, we love the process of making the show, and it's unlike anything else on TV. And getting to do this, to have that connection with an audience which recognises how special it is, is an incredible thrill.''
He and his fellow producers have been particularly touched by the extent to which the series has re-energised music and singing programs in schools. ''When you hear about how it has impacted on those programs, and on how teenagers who are struggling with issues of identity and how they connect with the characters on the show, it is humbling.
''I recognise in Ryan Murphy a great maverick spirit that makes a show like this possible and I'm constantly in awe of it.''
Glee
Wednesdays, Ten, 7.30pm
