Think 90s sitcoms and you think Friends. Then you think Seinfeld. Then you remember Frasier. Somewhere at the back of your mind, perhaps near Spin City, lurks Will & Grace, a sitcom about an uptight, straight-acting gay lawyer (Will, played by the straight-in-reality Eric McCormack); his shrill, insecure best friend (Grace, played by Debra Messing); and, thankfully for the humour quota, two supporting characters in the shape of Sean Hayes’s Jack (gloriously camp, perpetually uninterested in being an adult) and Megan Mullally’s Karen (chronic drunk, perpetually uninterested in being an adult).

Starting in 1998, its representation of gay men was seen as both a breakthrough – in 2012, then-vice president Joe Biden said it had helped educate America on same-sex marriage – and heavily criticised for presenting its gay characters as either unthreatening and neutered (Will) or the physical embodiment of what the predominantly straight viewership assumed all homosexual men were like (Jack).

Thankfully, there are enough thinkpieces retrospectively appraising Will & Grace to leave the real issue open to dissection: when did it lose its shine? The show’s central story arc – two very single old college friends turned roommates living unhealthily codependent lives in New York being charmingly berated by two responsibility-free daydreamers – worked for the first four of the show’s eight seasons, the spark between the quartet fizzing spectacularly. Confident in its lead characters, the early seasons relied sparingly on a small supporting cast, only adding in guest stars to add backstory (Debbie Reynolds as Bobbi, Grace’s mortifying mum, for example).

Season five ushered in two massive shifts, however. Firstly, the producers assumed one way of halting declining viewing figures was to chuck in as many celebrity cameos as possible. The fifth season alone featured toe-curling appearances from Madonna (as Karen’s flatmate), Demi Moore (as Jack’s old babysitter) and Elton John (as Elton John); while Janet Jackson and Jennifer Lopez were also crowbarred into later episodes. One person who should have remained a cameo was Harry Connick Jr, who smeared himself over the show’s latter half like Vaseline, dulling everyone’s shine. At some points, it wasn’t clear whether Connick Jr, as the catalogue model-esque Dr Leo Markus, was actually aware he was in a comedy, while at others he couldn’t hide his disdain for being in this particular comedy in the first place.

Introduced at a pivotal point, just as Will and Grace decided to have a baby together, Markus swept Grace off her feet, the pair eventually living together and getting married. Suddenly, the show’s central relationship went from two’s company to three’s a crowd, with the extra appendage a shiny deadweight with a boyband centre parting. Will and Grace’s platonic codependency was the heart of the show. Once Grace found someone else to help her navigate adulthood, the show started to unravel. It also put even more pressure on Jack and Karen to carry the comedy, their various scrapes becoming more ridiculous as the seasons progressed. Fingers crossed the producers learn their lesson for this September’s revival. No Harry Connick Jr, thanks.