"fast on the trigger"
A man with a gun goes into a bank and demands money.
Once he is given the money, he turns to a customer and asks, 'Did you see me rob this bank?'
The man replied 'Yes sir, I did.'
At this, the robber shot him, killing him instantly.
He then turned to a couple standing nearby and asked the man, 'Did you see me rob this bank?'
The man calmly replied, 'No sir, I didn't,
...but my wife did!