I mainly inspired by the Sonic Twitter Takeovers (you can find the audios on the official sonic twitter and their youtube) but the general idea is that the "character" runs the page for the day answering questions and maybe posting a bit of goofiness. The point being to get a little more than the usual reader engagement and give an opportunity for folks to interact with their "faves".
I personally think Twitter or Instagram would be a bit better for such a thing unless you keep the event limited to the comic (the walls on Tapas aren't really accessible to mobile and thus you lose out on some potential engagement esp, without notifs). It's practically the same as maybe a character QnA but I like the idea of a 'takeover' since it gives the involved characters a bit of "sentience" as they're engaging with the readers