I have an assain who is hired to kill a man. And she talks about where she is, but then it goes wrong. I don't know if I should either a.) Do a plot leading up to the moment on the plane or B.) Have her continue living life as if that was just another job and an intro into what she does for a living.

Both options are good, and could be really interesting. Downside is, if I do past, It could get tricky. But it could also make for some super awesome writing.

I'm confused as to the plot. Could you give a little more detail?

General advice I could give is, what fits the character? Is she a cold-hearted bitch who doesn't care? If so, maybe you should have her continuing life and the incident is just the introduction to the amazing story that will unfold. It would set up her "It's just another job" attitude.

But maybe you can combine the two. I'm throwing a random guess out there that she's gonna end up liking this guy, or somebody else in the process. If this is the case, start with her on the plane. Tell what's happening. Then go back to the beginning, give the story that leads up to this, and somewhere in the beginning have her at another job where she kills somebody and doesn't give a shit about it. Then you can work her morph her personality from heartless to loving. Of course you could make this work if she doesn't fall for him, too, that was just the first example I could think of.

[edit] That ping didn't work very well. :X