One must remain dignified and keep a stiff upper lip at all times, the age-old rules of royal etiquette must say somewhere. And absolutely no touching.
But even celebrities can slip up occasionally with all the excitement of meeting royalty. The same can also be said of certain members of the royal family. (We'relooking at you, Queen Elizabeth II.)
Here are 10 stars looking completely awestruck in front of Kate Middleton & Co., and vice versa.
1) Cara Delevingne meets Prince Charles and Camilla
Being spawn from aristocratic stock means Cara can guffaw as wide-mouthed and haughtily as she likes. (Plus, Camilla probably just shared a cracking gag.)
2) Reese Witherspoon meets Kate Middleton
Kate's so getting an invite to Reese's place in the Hamptons.
3) Keanu Reeves meets Princess Beatrice
Keanu's keeping cool as a cucumber (as usual), but Eugenie has gone as red as her frock. Not that we can blame her.
4) Beyoncé meets Kate Middleton
Queen Bey is basically royalty, so meeting the Duchess was no biggie.
5. LeBron James meets Kate Middleton and Prince William (and breaks the no-touching rule)
Forget that he's clearly breaching protocol with the cozy hug — those sweat patches should not be touching that expensive, dry-clean-only coat ...
6) Leonardo DiCaprio and Princesses Beatrice and Eugenie
A very, very young Leo looks completely nonplussed by meeting two princesses, but the same probably can't be said for Eugenie and Beatrice. (Watch that hand, Bea.)
7) Jennifer Garner meets Kate Middleton
We've just found ourselves the perfect actress to play Kate Middleton in a future biopic.
8) Nicole Kidman meets Prince William
"And my dance moves went a bit like this ... "
9) Tom Cruise meets Prince Philip
Chill out Cruise, it's not like you're meeting royalty or anything.
10) Jennifer Lopez and mother Guadalupe meets Kate and Wills
Because even famous people can sometimes be a little embarrassed by their parents.
11) Kanye West and P Diddy meet Prince Harry
We'd buy their music.