Kathryn Hahn had to wear a cooling suit underneath her 'WandaVision' costume because they shot at the height of summer

• Kathryn Hahn told Insider that she had to wear a cooling suit underneath her "WandaVision" costume.
• Hahn said that they were shooting scenes while the California wildfires took place.
• She said she would not have been able to shoot in her Agatha costume if it weren't for the suit.

Donning the witchy outfit for the first time was "a beautiful moment," Kathryn Hahn, who played Agatha Harkness in "WandaVision," told Insider.

Although the costume looked great, it did have its downfalls. Hahn, who was speaking to Insider about her new partnership with cleaning brand 9 Elements, told Insider she was forced to wear a cooling suit underneath the many layers of Agatha's costume.

This was "imperative" in order to film her scenes for "WandaVision" as they were shooting "at the height of the highest temperatures" while the California wildfires were taking place.

"It was very hot. So I had to wear a cooling suit under my costume, which is something that I had never heard of before, but it's like tubes of cold water," Hahn said. "Between takes my amazing costume assistant Beth would squeeze coils of cold water underneath my costume."

Hahn said that she actually rejected the offer of the cooling suit at first as she didn't want to wear another layer, but she quickly found that she needed it - or else she couldn't have filmed in that costume in such intense heat.

"There was no way I could have done it without it - it was imperative," Hahn said. "Apparently a lot of the Marvel dudes wear it as well, so that made me feel a lot better, but yeah there was no way I could have done it without it."

Agatha's costume change came after the character was revealed to be the big bad villain of the show in episode seven of "WandaVision."

Hahn then donned Agatha's witch-like robes, and the actress told Insider that the costume was the result of "a beautiful collaboration." Costume designer Mayes C. Rubeo, and the makers of the show wanted to "honor Agatha from the comic books but also put a modern look on top of it," she said.

"The costume designer, Mayes, is a genius. We really spent a lot of time creating that look together. I'm not going to say it was the most comfortable in the heat, but it was really well thought out and a truly beautiful moment to put on," Hahn said.

The costumes of "WandaVision" were one of the most celebrated aspects of the acclaimed show, with fans making clear their love for both Wanda's new Scarlet Witch look and the costume reveal of Teyonah Parris as Photon. Director Matt Shakman touched upon the show's costumes in an interview with Entertainment Tonight, saying that Elizabeth Olsen had a big impact on Wanda's new costume.

Hahn also touched upon one of the many fan theories that spawned from the show - the presence of Mephisto. Hahn said that while she has "zero idea" what Marvel's plans are for the comic-book character (if any), she "would not be surprised" if Mephisto crops up in future MCU projects after he was strongly teased in numerous "WandaVision" easter eggs.