Apparently, after the events of Her, Joaquin Phoenix went mad and is set to appear as the Joker. Or at least, the man before he turned into the Joker.
It's starting to sound a little confusing, isn't it? You thought Jared Leto - the vocalist of the band, Thirty Seconds to Mars - is the Joker. To clear things up: The Joker origin film is part of Warner's new banner of original DC films. Think of it as, a spin-off of the main DC Extended Universe. (If you think about it, the spin off should had been called the "extended universe." This allows Warner and DC to make multiple films of the same characters with different actors.
Also, my favorite theory in all of this, Jared Leto's Joker is actually Batman's third (or fourth?) Boy Wonder, Tim Drake. If you've seen the animated film, Batman Beyond: Return of the Joker, you'd get it.
The Joker origin film will be directed by Todd Philip, best known for The Hangover. So you know you're in for a treat of some dark humor.
Joaquin Phoenix is best known for movies Her, Signs, The Gladiator, and his finest performance yet, You Were Never Really Here.
There is no estimated time for the Joker film to come out but multiple online publications suggest it could be as early as 2019. Such a short time. I guess that means there won't be much CG in it.