I've worked in mortgage sales but not advertising. What are some good questions to ask the interviewer that will relate to the new job field?

Depends on what kind of advertising, print, radio, newspaper, new media? Generally you are very safe asking about the target group of your prospective media-who they are, what kind of money they make, what they spend their money on. Also what their (the media's) "cpm" is-cost per thousand-why they use an m for thousand i dunno. This is
probably the most crucial number to know and shows you've done your homework.

Also-do your homework.. Know the media-whom they market to, etc.

I assume this will be commission? If so it is nice to know what the last guy or last few guys brought down, and what they expect(ed) a salesperson to do.

A really good book for ad is The Accidental Salesman-his stats retrieval is too cumbersome for me, but some good anecdotes and sales help.

If you are outside sales, ask if there is a company car, car allowance, or mileage reimbursal. Cell phone same, if there will be overnight stays, expense account, etc.

Relate your mortgage sales as best you can to ad. Essentially you are selling the intangible in both cases. Bring this up. Ad people are really cognizant of the fact they sell something you can't see taste touch or smell, and ROI (return on investment) is rarely something you discuss with smaller ad buyers, because they don't track their sales very well in the first place, and asking them to break it down causes their eyes to glaze over.