Subtitled Folie à Deux, which translates to a 'shared delusion between two people', the movie takes us back to the chaotic world of Joaquin Phoenix's clown Arthur Fleck - joined by Lady Gaga's psychiatrist Harley Quinn in Arkham Asylum.
Despite only cameoing for a split second in the original Oscar-winning comic book thriller - Ethan was a guest on talkshow Live! with Murray Franklin - Theroux teased his character's return on Instagram this weekend with a picture next to director Todd Phillips.
"Midnight jokers #ethanchase #thatsawrap," read part of the actor's caption.
As for whether he gets more screen time in Joker: Folie à Deux, it's anyone's guess right now.
In conversation with The Hollywood Reporter two years ago, The Leftovers star Theroux weighed in on the possibility of coming back as Ethan, commenting: "All I can say is I haven't spoken to Todd in forever, and I have not approached him about doing anything for Joker.
"I won't say any more than that. Unless he has some brilliant way to weave in that small character, that would really be shoehorning a character into the movie that probably doesn't need to be there."
On how he initially landed the blink-and-you'll-miss-it gig, Theroux revealed that Phillips rang him up one random day with a conveniently timed offer.
"If I wasn't free, I wouldn't have done it, but I was free, so I said, 'Yeah, I’d love that. That would be super fun.' What's wrong with sitting around and giggling for a day with Robert De Niro and Todd Phillips?"
Watch: Joaquin Phoenix passed out on set of Beau is Afraid