Hi!
Eventhough I have no experience in job interviews, I believe in two things:
Never lie to the interviewer. If they discover that you're lying, that's it! You'll have hell of a time justifying that little lie! If you start lying, you'll have to put asside mind capacity to sticking to that lie so that you won't contradict yourself later in the conversation.
Never show that you're frustrated. They will, as all the other previous posts have implied, take in the information that you give them and then look at it the way they want to. A jump-seat ride can very well be a bad thing. The interviewer I saw (for my school application), told me I'd had a bad childhood in which my father and brother had been very physically violent towards me. This only because I'd told her that the relation between me and my father isn't really that good, but it's not bad either. What they want to do is to provoce you and confuse you. If you feel that you're being provoced, just take it easy and don't show your anger. There are some weird people in the buisness and they want to make shure that, if you have to work with a crew member posessing these charectaristics, it still will not pose a problem with the cockpit environment. It might seem a bit weird (perhaps because my English is really bad today!). But if you think of it, it makes sense. At least to me...
Think of all your shortcomings and try to correct them. Regarding the motivation part, just come up with all the reasons you can think of. Economics should be a good one!
Also show that you're aware of your "not so good sides". People that aren't aware of the situations in which their performance is lower than standard, will have a hard time being a pilot. If you know what I'm talking about that's a very big step towards being accepted by the interviewer (i think)!
In short my advice is this:
Don't lie
Never let yourself get provoced
Show that you know your faults and that you're doing something to make them better.
good luck and best regards/lns