Anything that makes people think more about how their character acts really. Asking for one or two of the character's embarrassing secrets or secret obsessions can help to make them feel more relatable and help others to reply.
Another way is to have a OOC mini-RP, completely taking the piss out of the main plot. Only one liners or vague replies, but a good pissabout to drum up some amusement and from there, inspiration.