Marcel's original shell was purchased in a hobby store. When Fleischer-Camp discovered he needed additional shells during shooting, he learned that, even if the shells were of the same species and purchased at the same store, the difference between them was strikingly noticeable. Eventually, he created additional shells using 3-D printing techniques.
According to various interviews from Jenny Slate and Dean Fleischer-Camp, the film's production process lasted approximately 7 years. An outline was written and dialog was improvised, recorded and assembled and this process was repeated and refined until the entire film's worth of dialog was completed. After storyboarding each scene, the live-action footage and backdrops were filmed, followed by the stop motion animation (on average, one full day of filming resulted in 5-10 seconds of actual run time), which was then finally composited onto the live-action footage.
With regards to working on the project with her ex-husband, Jenny Slate said, "In some ways, there are challenges that come up when you're working with someone, and your relationship is ... is making a big, big shift. But no ... It certainly wasn't dramatic or fighting or something like that. This is both of our favorite piece of work, I think, that we do, and are doing, and have done. So, I think we just wanted to keep our eye on that. We never talked about it, but I feel like both of us understood the doing the work and doing it well."
Marcel's grandma is named after Jenny Slate's Nana Connie, who appeared in the actress/comedian's Netflix 2019 stand-up special "Stage Fright."
The poem Nana reads just before the 60 Minutes interview is called "The Trees" by Philip Larkin.