To the unwitting spectator, the red carpet doesnt seem like a particularly funny place.
In fact standing up and being scrutinised in front of the worlds press sounds a bit daunting to us.
The pair was at the film festival to promote their new film, Inside Llewyn Davis.
Whats so funny? Thats no perfunctory, Im pretending to like my co-star laugh - its full on, side-splitting hysterics.
Over to you. Tell us on Facebook what you think Justin and Carey are laughing at.