Todd Phillips decided to make ‘Joker: Folie a Deux’ after finding that he and Joaquin Phoenix were "really sad" after finishing the first movie.

The director, 53, and 49-year-old actor - who plays the Joker/ Arthur Fleck in the picture - have returned to the dark DC universe with the follow-up to the acclaimed 2019 flick, and Phillips has now revealed he and Phoenix agreed to develop a sequel once the pair found they "didn’t want it to end".

Speaking to Entertainment Weekly, the filmmaker said: "After the first ‘Joker’, Joaquin and I were really sad. Like, really sad. We didn’t want it to end.

"Not only because we like working together, but because we didn’t want to leave Arthur. We loved Arthur and became attached to that character. Simply put, it was to spend more time with Arthur."

The movie follows the criminally insane Arthur Fleck (Phoenix) as he falls madly in love with Lady GaGa’s Arkham Asylum music therapist Lee - a version of DC Comics’ Harley Quinn - and the director explained he wanted to explore the titular character’s "light" and internal "romance" in the picture.

He said: "He had issues. Clearly. But, there’s a light, a beauty, and a romance inside of him.

"It was something Joaquin and I talked about early on; yes, he’s out of step with the world. However, there’s a romance inside of him, and there’s music inside of him."

Even though the romance can be "kind of violent", the filmmaker stressed the budding relationship between Arthur and Lee still had an element of warmth to it.

He said: "What happens when a guy who hears music inside his head finds love for the first time in his life? Maybe the music that he hears inside his head starts coming out. Why wouldn’t that music come out when he meets somebody who gives him the time of day?

"His biggest issue in the first movie, outside of childhood trauma, was a severe lack of love.”

While both Phillips and Phoenix were desperate to reunite for ‘Folie a Deux’, the filmmaker previously hinted the movie would be the final instalment in the franchise because the team felt like they’ve explored all that that world has to offer.

He told Variety: "It was fun to play in this sort of sandbox for two movies, but I think we’ve said what we wanted to say in this world."