Why so serious?

“Joker: Folie à Deux” has been given an R rating, according to the MPA’s daily ratings bulletin, for “some strong violence, language throughout, some sexuality, and brief full nudity.”

The film, starring Joaquin Phoenix and Lady Gaga, is a sequel to 2019’s “Joker,” which was also rated R.

An R classification is fairly rare in the world of comic book movies. In the 33-flick Marvel Cinematic Universe, only “Deadpool” and “Deadpool 2” were slapped with it.

Racy subject matter is slightly more common for the old DC Extended Universe (of which “Joker” and its sequel exist separately from): “Birds of Prey,” “Zack Snyder’s Justice League” and “The Suicide Squad” are all rated R.

Despite requiring adult supervision for any ticket buyer under 17, “Joker” grossed over $1 billion worldwide — the first R-rated film to do so. Phoenix won the Oscar for Best Actor.

In director Todd Phillips’ follow-up to his Best Picture-nominated “Joker,” Phoenix, 49, returns as creepy Arthur Fleck and is joined by Lady Gaga, 38, as Harley Quinn.

“Folie à Deux” will also star Zazie Beetz, Catherine Keener, Brendan Gleeson, Steve Coogan and Harry Lawtey.

According to Variety, the movie will be a jukebox musical with at least 15 cover songs. One tune is said to be “That’s Entertainment” from 1953’s “The Band Wagon.” 

“I think people will be surprised,” Beetz told the trade. “I don’t think it’s going to be what they expect, around it being musical.”

“We all sort of express musically and dancing in our lives day-to-day,” she added. “I think it’s going to work really well.”

“Joker: Folie à Deux” hits theaters Oct. 4.