In a lot of instances, I can see cool and discipline really intermixing. I can also see cool and vigilance intermixing.

scoundrels, doctors, and explorers get cool as a class skill.

Scoundrels I can see. Keep your cool, keep a straight face...

Doctors. Eh? Seems more like discipline to me. A disciplined mind. A steady cutting hand. HOWEVER I can see the cool coming into effect here: Doctor, my legs just got blown off -- ahhh!!. Doc better keep cool. So I guess I can see it.

Explorer. Mmm...

So yea. I get what they were trying to do. I just feel the skills have some overlap. So when overlap occurs, GM fiat.

Cool isn't so much about preparing an ambush as it is about not flinching when the time comes to pull the trigger (poker face for your gambler, steady hands for your surgeon, and quick draw for your gunslinger).

Considering that line, how can there be any justification for the Assassin not having Cool as a Career Skill?

Because each spec only gives up to 4 new career skills (Assassin gives 3, because it 'doubles up' on Ranged (Heavy)), and the ones given by the Assassin spec are at *least* as core to the concept as Cool. Basically, they had to draw the line somewhere, and decided that Cool just didn't make the cut.

That said, it's not terribly expensive to get a rank or two in a non-career skill, even for non-humans, so it's not that big of a deal.