Under Part 135 operations, a co-pilot only needed to have a knowledge of the aircraft systems, performed some single engine work and made three take-offs and landings.
Some less scrupulous operators took a flexible approach to this.
A friend of mine recounts his newly rostered first officer climbed into the right seat, looked around in wonderment and said "Golleee!"
But the large 135 operation I flew with in the US ensured that all new hires were FSI trained and had spent several hours in the air with a training captain before being let loose on line.
So we had few problems.