On the “Always Sunny Podcast” episode “The Waitress,” co-star Mary Elizabeth Ellis, who plays the unnamed waitress who Charlie (Day) pines after, revealed that she had never really met DeVito before she filmed with him in “Mac Bangs Dennis’ Mom.” In the episode, Mac (McElhenney) sleeps with Dennis’ (Howerton) mother, which leads to Dennis trying to sleep with Mac’s mom to get back at him. It all leads to the Waitress having sex with Dennis’ supposed father, Frank, in an attempt to upset him. I their first scene together, Frank acts pleased that he just had sex with the Waitress, while she is clearly distraught. When Ellis approached DeVito to film, however, he simply grunted at her. It was part of his warm-up, as Day explained:
“Danny has a thing where right before the camera rolls, he’ll usually improv a line or two in. […] Make like, if he’s being lascivious, he like gets into that. Or if he’s being angry, he might be moody about something. […] It’s not like a Method thing, but it’s just like a little warm up. So you don’t just go in, ice cold.”
Frank is being pretty lascivious in the scene, which would be extremely unnerving to encounter if you didn’t know DeVito and it was your first time on set with him. Thankfully for everyone involved, Ellis being freaked out made the scene even better, and she was totally fine afterwards once she got to know DeVito a little bit.