In modern comic book movies, cameos are not only a regular source of fun for audiences, but also commonly used as a storytelling tool. With series often planned out in advance, filmmakers have sometimes used special appearances by known actors as a means of teasing future chapters – with the perfect example being Samuel L. Jackson's debut as Nick Fury in the first Iron Man. All that being said, if you happen to be a person who is expecting to see more from Justin Theroux in Joker 2 following his extremely minor part in Todd Phillips' villain origin tale, we have some bad news for you.
With news recently circulating about Joker 2 taking steps towards becoming a reality, The Hollywood Reporter recently asked Justin Theroux if he was in any way "vying" for a role in the project, and the actor essentially shut that talk down. According to the Mosquito Coast star, he hasn't been in contact with Todd Phillips in a long time, and he doesn't personally see a path towards bringing his character back. Said Theroux,
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.
In Joker, Justin Theroux plays an actor named Ethan Chase... but if you didn't know that it's not exactly a sleight against your comic book movie knowledge. It's a case of a cameo being less of a "blink-and-you'll-miss-it" appearance and more of a case of "squint-or-you'll-miss-it." As seen in the screenshot at the top of this article, he only shows up for a few seconds in the film, and he's only ever featured on a television screen as a guest on an episode of Murray Franklin's talk show (Arthur Fleck watches and imitates him as he prepares for his own appearence).
For what it's worth, there is a bit more to Justin Theroux's Joker cameo beyond a familiar face being featured in a sly way. In Todd Phillips' 2010 movie Due Date, Zach Galifianakis plays an actor named Ethan Tremblay who travels across the country to move to Los Angeles – and his plan is to perform under the stage name Ethan Chase.
As for what's happening with Joker 2... there isn't much to report. Todd Phillips has apparently made a deal that will see him return to co-write the sequel, but we have no idea what the movie will be about, or how long it will be before it's a project that is ready to go into production. There's a lot of anticipation for more information, however, so we'll surely get more updates soon.