https://toby.fangamer.com/comics/grillbys
In this comic from the undertale/deltarune newsletter there are profiles on each person that occasionaly shows up at grillbys. Including Grillby himself.
So here is a quick list of things that I personally find interesting or funny that are mentioned in this comic (tho I'm probalby a little biased here):
- the drunk bunny apperantly occasionaly flirts with sans and Grillby
- apperently once asked Grillby to toast marshmallows in his mouth for her, which resulted in him no longer even touching her food
- Grillby can, in fact, talk but only seems to do so in rare occasions (I always thought he can't speak)
- two children made a snowman in the shape of Grillby because he couldn't do it himself
- sans intentionally translates the things that Grillby say so bad that Grillby had to step in
- apperently sans does have fried snow, that he also sells since he seems to sell it to the mouth monster
- sans canonically gets along with Grillby well, to the point where he does oddjobs for him and even runs the bar on the rare occasions where Grillby steps out.
Anyways I would really recommend reading the comic