Producer Sho Tanaka, who also produced Overlord, read the light novel volumes in depth once he learned they were being published, already anticipating an anime adaptation.

Author Carlo Zen advised the production team that television anime has its own approach and asked them to focus on entertainment rather than trying to fully reflect the philosophical and religious themes of the original novel.

Director Yasumi Kamimura considered giving Tanya's inner monologue to a male voice actor, but producer Tanaka suggested the monologue should also be voiced by the young-girl voice actress, leading to the final decision to keep it all with Aoi Yuki.

Aoi Yuki said that from her audition she felt performing Tanya with more 'monster' qualities than 'little girl' qualities would work best, aiming for a ratio of about 30% little girl and 70% monster.

The film's climactic one-on-one confrontation between Tanya and Mary intentionally uses no music at all, a deliberate choice by sound director Yoshikazu Iwanami to let the audience reflect on the film's themes instead of following a conventional dramatic score.

Recording for the theatrical film began around September 2018, unusually late for a theatrical anime, and Aoi Yuki recalled with co-star Saori Hayami that the sessions took a long time to actually start.

During a stage greeting, Mutsumi Tamura (who voiced new character Vivi) admitted Aoi Yuki's performance scared her, to which Yuki replied that it wasn't her that was scary, but Tanya.

The film ranked first in Filmarks' opening-day satisfaction ranking for movies released in the second week of February, and audiences frequently praised the film's sound quality according to sound director Iwanami.

The Isekai Quartet crossover anime brings together chibi versions of characters from four KADOKAWA franchises: Re:Zero, Overlord, KonoSuba, and Saga of Tanya the Evil.