[5] After reading the film's script, Hildur was asked by director Todd Phillips to write some music based on her feelings of the Joker screenplay, which she was inspired to do so as it resonated with her.

[6] She sent Phillips a sample of her composition which included melodies that were very simplistic and monotonic as a means of conflating the film's gritty tone with that of protagonist Arthur Fleck.

[17] Unlike her previous collaborations, Hildur began composing Arthur's theme during the pre-production of Joker, with Phillips initially hiring her simply to come back to him with some ideas rather than a full score.

[18] Phillips asked if she was interested in writing some music just based on her feelings of the script: "He didn’t really give me any instruction, as such, he was just curious to hear what I felt".

[19] In an interview with Deadline Hollywood, Hildur remarked that the script resonated with her and that Phillips believed she captured the atmosphere of Joker, based on the sample she sent him.

[25] Additionally, Hildur spoke of feeling there was greater room to work both thematically and imaginatively, based on the fact Joker was fiction, in which she added: "It was more an emotional and, above all, surprising search for the character, someone with whom everyone in my generation grew up, but whose motives are unknown to anyone.

[26] Phillips' Joker allowed for Hildur's imagination to "run wild a bit more", in that she decided to write something orchestra-led, though this time it was the process that was unconventional.

[25] She spoke of sitting down with her cello and attempting to find her way into Arthur's voice and into his head, eventually doing so: "[...] it was a really strong, physical reaction that I got.

[28] Phillips changed his mind - playing Hildur's composition over each and every take, in which Phoenix's Arthur starts his metamorphosis dance,[28] by which Sher added: "The score was such an instrumental part, not just to Joaquin's performance, but to the camera operating, to the sort of energy in the room, and to make that scene really come alive".

[35] Phillips spoke of how Phoenix just started to dance to Hildur's music, while adding: "It was just me and him alone in the bathroom, there's 250 people on the crew waiting outside.

The preparation was in studying movement and dance during rehearsals, but what came out of that piece of score was a turning point for the character, and for me and [Phillips] working together [...] and understanding Arthur".

[39] Hildur wished to expand simple and monotonic melodies within the orchestration around Joker's evolution not with chords or any complicated music, but instead with texture that she felt resonated with the melancholia of this Arthur.

[52] An edited version of The Guess Who's "Laughing" in an Instagram post by film director Todd Phillips as foreground music of a camera test for Joker, which offered the first look of Phoenix in makeup.

[93] David Rooney of The Hollywood Reporter wrote that "all this is rendered even darker by the disquietingly melancholy mood of Hildur Gudnadóttir's brooding orchestral score, which cranks up into thunderous drama as the chaos escalates".

[95] Kayleigh Donaldson of Syfy described Gudnadóttir's score as "beautifully haunting", whilst remarking: "[...] such a refreshing change from the frequently derivative music heard in comic book movies".

[96] Terri White of Empire wrote: "Mention must be made of Joker's cello score by [Guðnadóttir] — mournful, dark and fractured [...]"[97] The Economic Times commented that "accompanying the visuals is an amazing sound design, Hildur Guðnadóttir's soulful background score and powerful music pieces".

[100] John Lewis of The Guardian wrote: "The soundtrack for the film Joker is a fine showcase for Icelandic composer [Guðnadóttir].

[101] James Morton of Impact wrote: "The score by [Guðnadóttir] is reminiscent of Jocelyn Pook's music for Kubrick's Eyes Wide Shut (1999), though deeper and slower burning.

[103] For Film Ireland, Michael Lee stated: "[...] all this is elevated by Hildur Guonadottir's menacing score, which seemingly ignites the embers raging within Arthur's heart".

[105] Critic James Verniere for The Boston Herald wrote: "The film's plaintive music by [Guðnadóttir], another departure from usual, is a huge part of the impact of Joker".

[106] André Hereford at Metro Weekly stated that "there isn't a bum performance in the movie, and composer [...] Guðnadóttir's strings-laden score sounds especially attuned to every one of them.

[107] Chuck Koplinski at Illinois Times praised Phillips' usage of Guðnadóttir's "thunderous score", allowing it to underscore the obvious betokens no great faith in the audience's ability to keep up.