"Joker: Folie à Deux" was always going to be a flop, and even members of the cast knew it ... this according to one of those very actors.

Tim Dillon -- a comedian who appears in the critically-reviled film -- stopped by "The Joe Rogan Experience" earlier this week ... and, during the three-hour discussion, the two chatted about TD's new flick bombing at the box office.

Dillon's unequivocal here ... calling the movie literally the "worst film ever made" -- saying he think the creatives behind it got cold feet when there was pushback from film critics on some of the first film's themes.

TD basically says he thinks the filmmakers overcorrected ... 'cause they didn't like their film being called out for depicting male rage and being loved by the incel community -- that they went too far and decided to simply put Joaquin Phoenix in tap shoes.

Dillon tells stories of sitting with other actors in minor roles and often asking, out loud, what the heck was going on with this film. He says others were shocked 'cause the movie seemed to have no plot ... adding it was sure to flop at the box office.

Dillon plays an Arkham Asylum prison guard in the movie ... which absolutely did bomb at the box office -- making just over $200 million globally. The first film made over $1 billion and netted Phoenix his first Oscar.

Critics hate the flick too ... with the movie getting just a 32% on review aggregator Rotten Tomatoes.