Take on the role of your main character, and have a conversation with a friend or an unsuspecting stranger 😉
Take on the viewpoints and attitudes of your character. It would be best to try to get into some sort of discussion in which you do not agree so that your character can defend himself, thus revealing his personality and motivations more fully. Actors could use this prompt as well as writers!
Full Circle Homeschooling 
Lol, my sisters have to beg me not to do this outside the house…
Years before I thought of myself as a writer, I was a habitual roleplayer. Middle Sis and I went through dozens of games where we were scores of characters each. Some we adapted from books or TV, many we made up from scratch. Baby Sis wanted to play, too, but her grasp of the concept was limited, so we, ah, kinda excluded her a lot. But now that she’s older and easily as cool as I am, she gets her share of laughing at me with my characters (with their author obviously *right there*… no respect, I tell you).
Heh, heh. I almost labeled this one as a prompt disguised as role-playing, but with the bad rap that role-playing has, I opted for its current title. Do your sisters write as well? Sounds like you had tons of fun!