Sorry captain!
Interesting report in this month's pilot magazine.
An off-duty f/a was on a US Air 737, and witnessed a wet start ie flames out the back of number 2 engine on startup.
On informing the duty f/a that there was an engine fire, the pax were immediately evacuated through the rear exits & slides.
Flight deck were completely unaware of any abnormalities until the ground staff told them that all their pax were on the tarmac!