Hollywood movies often bring together unlikely combinations of co-stars (often with equally mixed results). But few movie pairings have ever been as unlikely as 1957’s The Prince and the Showgirl. Telling the story of a Balkan prince regent who falls for an actress and dancer while on a visit to England, the movie paired British stage legend, Sir Lawrence Olivier, with Hollywood pin-up Marilyn Monroe. And perhaps understandably, given just how mismatched its stars were, the movie’s production was not an easy one.

The story of The Prince and the Showgirl had started out as a play, which Olivier had starred in with his wife at the time, Vivien Leigh. Keen to adapt the play for the screen, Olivier—who went on to both produce and direct the movie—found that rights had already been bought by Monroe’s production company. This development soon frustrated Olivier, and after Monroe was cast in the movie and filming began, the pair’s relationship quickly soured.

Monroe was reportedly late to set every day, rarely hit her marks, often forget her lines, or changed line readings from one take to the next and was near constantly consulting her acting coach between takes. Olivier struggled with Monroe’s lack of professionalism and ended up treating her appallingly—often calling her names, bad-mouthing her performance behind her back, and even opening a sweepstake on how many takes she would ruin before finally making a good take. When Monroe heard that the crew were taking bets on her performance, however, she decided to take matters into her own hands.

One day, knowing that she had a particularly difficult scene to film the following morning, Monroe studied immensely hard the previous night. She turned up to set fully prepared and nailed the scene in a single take, walking off the set and closing a door behind her. Moments later, she reopened the door, stuck her head through the gap, and declared, “Pretty good, huh?”

As unscripted as the line was—and much as it was intended as a slight at all those on set who doubted her talent! —Olivier liked it and knowing Monroe had well and truly got the better of him, he kept it in the final cut.