The actor – who left Downton in series three – was asked by Susanna if he'd had to "beat off a lot of American men" to win the role in his new movie The Guest.
An innocent enough question, right?! Well, apparently not to naughty Dan, who started giggling and took a while to stop!
Crew members in the background could also be heard sniggering.
Watch Dan's giggles (from around 1:30)
A bemused Susanna continued: "Why does that make you giggle? Did you not have to beat them off with a big stick?"
Cue more giggles from Dan!
Later on, showing off his sense of humour, Dan posted a link to the interview on Twitter alongside the caption: "The Great British Beat Off in full…"
Susanna responded: "You are a terror."
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