Amie Donald performed any of M3GAN's scenes that called for physical movement the puppet could not do. She also performed all of her own stunt work. Donald received movement coaching from Jed Brophy and Luke Hawker in portraying M3GAN's agility. On set, Donald wore a static silicone M3GAN mask created by Morot FX, and this was later replaced by a CGI version of M3GAN's face to match that of the animatronic.
The film was originally R-rated by the MPAA. While editing, it was evident that it could be a PG-13 rating by altering a few scenes, and attract a larger audience. Due to the trailer's popularity among teens, some scenes were reshot to tone down the violence and obtain a PG-13 rating.
Adrien Morot and Kathy Tse of Morot FX Studio created an animatronic puppet version of M3GAN that was used for dialogue and close-ups. There was also a second animatronic used for certain scenes, as well as a posable stunt version of M3GAN that wasn't puppeteered. The animatronic M3GAN was puppeteered via a variety of techniques, which included radio-controlled facial expressions performed by Adrien and Kathy in tandem, automated lip-sync for the dialogue (temp tracks were provided during filming by New Zealand actress Kimberley Crossman) and a puppeteer physically moving M3GAN's head and body.
M3GAN's voice is modelled, in part, from another famous artificial intelligence-gone-rogue, GLaDOS, from Portal (2007). This is most noticeable during scenes when M3GAN's voice is auto-tuned in order to sound 'glitchy'.