For most people, the face is the starting point before getting around to other phsyical essentials. Type-casting is something all actors know about - having the right look is what really matters. Also, attitude is usually written in the face.
Whatever scenario you have in mind, whether you imagine being involved with the characters or just watching the action, having a strong image as a reference point can make the scene really come alive.
Choosing types you prefer to fill each role in a scenario can, as a process, be stimulating in iteself. Finding faces that you'd like to see involved in the action might lead to you building your own file of lusty players for future reference.
Particularly when you're inventing a scenario from scratch, you're in the director's chair - directing the action. So - director's get to select the 'talent'.
I discussed suitable actors to fill the two parts - and the Mechanic might be either ...
... or ... or ... or even
All a matter of