When you get the contract in your hand, that's when they're serious. Until then, you could be left treading water whilst someone signs off on what the external message should be. In the absence of information to the contrary, keep jumping through the interview hoops. Only if you're obviously being messed around should you be the one to call it off - in the run-up to Christman, interviews can be moved around for honest reasons (holidays, interviewers getting colds, etc.) so don't panic yet.