The success of Supernatural rests in large part on the shoulders of these two actors (and Misha Collins). For one thing, the show is about two brothers and their love for each other, which is so great they will literally move heaven and Earth for the other one – if the two lead actors didn’t have chemistry with each other, it wouldn’t have lasted half a season. But they do, and they also both have bucketloads of charisma, giving them the ability to keep the audience engaged year after year after year, always coming back for more of Sam and Dean and their emotional traumas.

It changes, but it stays the same…

If there are two things certain to kill off a long-running show, one is changing too much, and the other is staying exactly the same. Changing too much risks alienating an audience who enjoyed the show as it was when they fell in love with it. Too many changes that are too extreme can make people long for the early days and switch off. On the other hand, not changing at all can be equally off-putting. A show that does exactly the same thing week after week for fourteen years is not going to hold its fans’ attention forever.

Supernatural has tended to get the balance right here, mostly through carefully balancing arc plots and standalone stories. On the one hand, the show still remains, at its core, about Sam and Dean, hunting monsters in their Impala (a brief flirtation with hiding the Impala thankfully only lasted one season). Even viewers who disliked the introduction of Collins’ Castiel in season four are still regularly served standalone episodes in which no other regular characters appear, and Sam and Dean hunt and kill/exorcise/otherwise render harmless a monster. Sure, fans talk about missing the glory days, as they will when watching any long-running show, but ultimately, the core of the show is still the same.

Other aspects of the series, however, have changed drastically over the years. The arc plot built up to a finale at the end of season five. Since then, following the departure of creator Eric Kripke, different seasons have had different focus points, usually relating to angels and demons in some way. Most significantly, season eight gave Sam and Dean a physical base of operations beyond their car, and opened up new story opportunities as they explored their Men of Letters heritage. And of course, as mentioned above, other cast members have come and gone, usually dying horribly and only occasionally coming back.

A different type of Very Special Episode…

Supernatural is far from the only show to feature regular format-bending episodes – nearly every SFF show that runs for a significant period of time will start playing in its sand-pit a bit by doing a few weird and wonderful, experimental episodes like musicals, Rashomon episodes, alternate universe episodes, meta episodes about the show as a TV show, and so on. Supernatural just happens to do this type of episode very, very well, making them reliably hilarious and inventive.