If I can cast magic circles basically wherever I want in a certain radius, what's stopping me from casting magic inside people?

“anybody want a drink?”

“sure I’m thirsty!” exclaimed my next unsuspecting victim

I calmly walked over and poured ink directly out of my fingers into their mouth

I’m very sorry I subjected you to this but it had to be said (you can tell I want that flaunt feature can’t you)