AS FRESH AS A CUCUMBER

WHEN YOU ARE AS FRESH AS A CUCUMBER MEANS THAT YOU ARE RELAXED AND CALM, INSTEAD WHEN EVERYBODY IS EXPECTING YOU TO BE NERVOUS.
HE WENT TO THE TV INTERVIEW AS FRESH AS A CUCUMBER. HERE IN SPAIN WE HAVE GOT THE IDIOM: AS FRESH A A LETTUCE. I THINK THEY HAVE THE SAME MEANING.
SOMETIMES CAN BE POSITIVE LIKE THE MEANING WE HAVE JUST EXPLAIN, BUT SOME OTHER TIMES, CAN BE NEGATIVE:"HE TOOK THE COFFEE WITH OUT PAYING IT, AND HE WENT AS FRESH AS A CUCUMBER." REMEMBER THE TV SERIE: THE FRESH PRINCE OF BELAIR.