I feel like one way to ease adults into comics can be to go with the non-fiction route. If they eyeroll and think comics aren't serious, I'd put Art Spiegelman's MAUS in their hands, or the works of Joe Sacco, like Safe Area Gorazde.
They're both works of non-fiction that deal with very heavy topics (Holocaust survivor's memories+generational conflict, and the Bosnian War, respectively) in a serious way - Joe Sacco's comics are basically in-depth journalism in comics-form. If the potential comics-readers are the type to prefer their literature a bit more serious-faced and with literary aspirations, this is a good place to start.
If you want to go non-fiction, but make it a little softer, I recommend Pyongyang by Guy Delisle. It's non-fiction, it's serious (it chronicles his time in North Korea, working in an outsourced animation studio), but it's drawn with a more cartoony, softer style, and not afraid to crack jokes.
If you have someone more inclined towards fiction, it's not a bad idea to put an anthology-volume in their hands, just to show them the range of what's possible. I'd recommend any volume of the Flight anthology.
And just in general, for anyone who doesn't mind a bit of fairytale inspiration, I believe you can't go wrong with reading Linda Medley's Castle Waiting. It uses familiar characters from fairytales, but tells a fresh, new and interesting story with them.