The designs of the Tulpar crew are all loosely based on characters from popular horror media. Two examples that the developers themselves confirmed in a diary post are Daisuke and Anya, who are respectively based on Ryosuke Kawashima from the 2001 Japanese horror film Pulse and Wendy Torrance (as portrayed by Shelley Duvall) in the 1980 film adaptation of Stephen King's The Shining.

According to a Q&A hosted on the game's Steam page, every character in the game is voiced by audio designer Martin Halldin, including Curly, Daisuke, Polle, and "anything that isn't an advert in the tv sections". In the case of Polle's voice, the intention was to have it feel like they were pulling voice clips from random advertisements to string a sentence together. To that end, the words were all recorded individually and cut into sentences during post-production.

Mouthwashing uses a visual style inspired by games released on the original PlayStation, with low-poly visuals and a retro style. The game's art designer Johanna Kasurinen was introduced to this style by Puppet Combo, an indie developer that frequently uses it for their games as far back as their debut title Babysitter Bloodbath. While not promoted as "PS1-style", this visual style gained traction over time, with Kasurinan stating:

In a developer Q&A posted on the game's Steam community page, the staff at Wrong Organ revealed that Jimmy was named after a person repeatedly mentioned in the 2023 song "A&W" by American alternative pop singer Lana Del Rey. In addition to inspiring Jimmy's name, the song may also serve as the basis for Spoiler:Jimmy's role in the game as Anya's implied rapist, given that "A&W" describes "the experience of bein' an American whore" and features multiple passages where Del Rey repeats the line "Jimmy only love me when he wanna get high."

In a conversation with EIP Gaming writer Dylan Graves, producer Kai Moore cited horror films such as Alien, Event Horizon, Sunshine, The Thing, and Pandorum as influences on the game's bleak, paranoid tone and isolated setting. However, Moore also countered the association of these kinds of horror stories with monsters, stating that he sought to make the player question the definition of "monster," particularly in the context of Jimmy's statement at the start of the game claiming that Curly willingly crashed the Tulpar.

The notice that Curly receives from Pony Express during the flashback sequences is fully written despite the game's low resolution making it illegible in normal circumstances. Hacking the game to remove the resolution filter reveals that Spoiler:Curly was due to be promoted following the layoff of the Tulpar crew, a detail that is otherwise never revealed in-game apart from vague mentions of Spoiler:Curly being the only crew member who can financially support himself after the layoff.

The game's main menu was originally designed around various monitors similar to those seen around the Tulpar. While this was replaced with a more plainclothes design in the final game, the original layout is still present in the same screen. However, it is hidden out of the player's view and blocked out by various gray boxes. In an interview with YouTuber Derek "Shesez" Forte on his series Boundary Break, developer Jeffrey Tomec stated that the original idea was that the main menu would mimic Curly's medical equipment, fitting how the title screen displays Curly lying in the medical bay. Each option would be displayed on one of the screens, and when the player selected an option, the camera would zoom in on the screen. However, this idea was abandoned due to concerns that the game's deliberately low resolution would render the text illegible.

In an interview with The Washington Post, art and narrative designer Johanna Kasurinen stated that Spoiler:Jimmy's dilemma of constantly making the crew's predicaments worse by trying to fix them was inspired by the development team facing a similar ordeal during a previous project. According to Kasurinen, Spoiler:the project was dead in the water well before the group decided to terminate it, yet they persistently tried to keep it going, stating it was "that state of a team of people trying to convince ourselves, 'We can still fix this if we just try harder,' when everything was already limping and on fire, that eventually planted the seed."

Similarly, the characters of Jimmy and Curly were based on people who were common in the workplace and other social environments, with Kasurinen stating that the developers wanted to both express this relatability and challenge players to see what elements of their own selves parallel the two, particularly Jimmy. Kasurinen noted how Spoiler:although she had never committed the kinds of atrocities that Jimmy does throughout the game, she still found resonance in her own experiences with jealousy, insecurity, and other "awful, clinging emotions" and "uncomfortable things, about aspects of myself or others that I find distressing, shameful, contradictory. Things that I think a lot of people can identify with but aren't usually put into words."