Thankfully, Robbie was told in the early days of her career, Get Out's Kaluuya, however, wasn't so lucky.
"I didn't know that," Kaluuya exclaimed, "apparently it's bad etiquette to put your tongue in."
Robbie, Janney and Vikander all burst out laughing as he continued: "I didn't know that. I was just doing it because I thought, 'We're keeping it real.'
"And then someone said, apparently if you fancy them, you put your tongue in. That's what someone told me and I was like, oh shit."
Margot, once composed, revealed that thankfully, a kind soul let her in on the no tongue rule before she went around accidentally flirting with too many big stars.
"Someone told me early on, thankfully, 'Don't stress about it, but no tongue.' And I was like, 'Oh, got it.'
I always say that to other people, like 'Yeah it's all cool, but no tongue,'" she revealed.
Graham Norton asked the question everyone on the couch should've been asking: "Why wouldn't someone tell you that?"
Seriously, isn't this something that should've been covered in acting school?
Kaluuya went on to say he didn't find out until years later, "this is just a random conversation years later. My life's a lie."
The stars also noted that in America people pass out breath mints before a kissing scene - something that's apparently not done in the UK and Australia.
“Everyone’s so germaphobic in America,” Margot noted.
Watch the hilarious conversation below.