When setting out to make Joker, Lawrence Sher and director Todd Phillips decided to go for an intimate, 70's style of movie that's character-driven, rather than the large scale, elaborate looks typical of today's comic book superhero/villain films. Joaquin Phoenix had some leeway to improvise and play with the character. Larry and Todd Phillips' background of working with comedians on movies such as The Hangover made them feel comfortable with improvisation.
The Cinematography Podcast