According to Webster, crucial is “important or essential as resolving a crisis, decisive.”
Critical means “being at a turning point or specially important juncture” or “relating to an illness or condition involving danger of death.”
They are not synonyms. "Critical" bumps it up a notch. The something you are describing is of vital importance, possibly a matter of life or death.
Strunk and White’s The Elements of Style has a great explanation of the difference between nauseous and nauseated.
“The first means ‘sickening to contemplate’; the second means ‘sick at the stomach.’ Do not, therefore, say ‘I feel nauseous,’ unless you are sure you have that effect on others.”
“Critical.” Webster’s Ninth New Collegiate Dictionary. 1991. Print.
“Crucial.” Webster’s Ninth New Collegiate Dictionary. 1991. Print.
Strunk Jr., William, and E.B. White. The Elements of Style, 3rd ed. New York: Macmillan, 1979. Print.
Up First for Flash Fiction Fridays
“Bashful Blueberry” by Karen Cantwell, author of Take the Monkeys and Run: A Barbara Marr Murder Mystery.