For assassins, start them out as saboteurs. Put a spy in the target city to determine the easiest targets (usually religious buildings). Two successful bombings gets you the "Plotter" trait. Two more gets you "Schemer." Four more gets you "Grand Conspirator," for a total of +3 skill. Also, you will most likely have picked up at least one (probably more) ancillaries while blowing stuff up, for a total of about 5 skill before you have tried killing even a single human target.

BTW, does anybody else giggle themselves silly when they click on their HRE assassin and he says:

"Here iss a message from se Kaisa."

or

"Don't schtruggle; you're only making it verse."

I only wish he would say "Vould you like to touch mein monkey? Touch it!"