If you have an interview, it means that they like you enough to take you. The interview will be mostly to see if you and the prof think you can work together for several years by interacting in person. I'd say it's very much a deal breaker. GRE marks, GPAs, whatever, won't help if personalities simply clash.
Questions vary depending on who's asking them. Your prospective prof will have probably (hopefully) read your application material several times over. Arguably the most important question, and one you better have an answer to, is "how does what you want to do fit in with what I'm doing?" Be prepared also for things like "what research projects do you have in mind?" "what sort of things do you want to work on here?" "any ideas for your master's dissertation?" "What do you look for in a mentor? In labmates and colleagues?" There can also be specific questions about any research you already did.
The other profs should have read you material, but will be considerably and understandably less familiar with it (they're focusing on people who want to work in their lab). You might get some of the really kind of pointless and formulaic questions ("Are you a member of any professional organizations? What have you done with that?" "Why this school?"). They might be interested in what you want to do while you're there-- TA for them, maybe. They'll ask you if you have any questions. Always do. Even if you have to ask all 3 of the profs the same question about funding just to "get everyone's opinion on the topic." Sometimes questions get a bit more interesting... "Why counseling psych and not clinical" (I got that three times), "What kind of statistical analysis are you doing on your senior thesis?" And then some of the crazy ones will ask off-the-wall questions. But, pressure interviews (the prof being intentionally rude or dismissive) seem WAY less common in psych than in Med.
My interviews all took an entire day-- program orientation and breakfast, some interviews, lunch with the students, more interivews, tour. There's often a get-together hosted by a current student the night before or of the interviews.
Every other interviewee will be wearing a suit, so wear one if for no other reason than not to be the odd one out. But, my luggage (with suit) got lost on the way home from one interview and I had 1 day till the plane left for my next one. I got my luggage back in time, but when I related that story to my prospective prof he laughed and said no one would care if I showed up in khakis and a nice shirt.
Also, one point-- your prospective prof might have more than one person who's getting an interivew for one opening. Which means you're in direct competition with someone else in that room for getting in. Be prepared for that by finding out who that person is on SDN and getting alllll the dirt you ca-- I mean, by being polite and courteous the entire time (I did interview at one school where they had two interview days, and didn't schedule anyone vying for the same prof on the same day. Nice of them).
About your MA question.... based on your other questions, I assume you have not applied yet. Is the program you're looking at structured with a seperate MA and PhD program? I ask because a very large percentage (I'd say the vast majority, really) of programs do direct-admit for PhD. So, after your BA you'll apply to the PhD program and snag a MA along the way. Funding is usually more secure in a PhD direct-admit program for first years than it is for MA students, too.