Photographers don't have the greatest of reputations at times. We've all heard of guys buying a camera just to be able to say "I'm a Photographer" in an attempt to get close to hot girls. They're referred to as GWC's (Guy With Camera). Models are wary of this, and it's unfortunate, but even decent photographers can inadvertently get this label slapped on them. Why? Social skills. You've got to have them finely tuned to operate in this field.
Here's 6 five things that might seem innocuous but you shouldn't EVER do at a shoot:
1. Touch the model without asking for permission.
If there's something wonky with her clothes, tell her, don't go up and fix it unannounced. If you want a certain pose, describe or mime what to do, don't go up and physically move her in to place like a mannequin. Yes, she might look tired after the shooting is done, but that doesn't mean you can give her a backrub or the like. Clear? Unless it's explicitly made OK by the model, no touching!
2. Ask her to remove more clothing than was originally agreed upon.
Some models are OK with nudity, some are only for increased pay, some not at all. If you don't ask in advance, by pressuring her at the shoot to remove extra clothing (and trust me, even if you think you're only asking, if you do it while at the shoot it feels like pressuring from the model's perspective) you risk coming off like all you care about is seeing more flesh. Not only that, but when the refusal sounds forth, the rest of the shoot will be awkward.
3. Use sexual phrases.
You're snapping frames off, and the model hits on a really nice pose. Don't say "that's so sexy", even if it is- try "oh, that's good, hold that for a second". It's generally a good idea to stick with platonic praise, as there's less chance of the subtext sounding like "I want to get into your pants". Saying things like "that's a great angle for your face" instead of "you look so hot" will do wonders.
4. Imbibe alcohol or other drugs before or during the shoot.
Getting buzzed beforehand is unprofessional. Swigging liqueur during is even worse. I'm assuming the extrapolation out to harder chemicals is obvious here too.
5. Watch the model change.
Give the model some privacy. If at all possible, have a dressing room for the model to change in. Even a curtained-off area in a corner works. Although many models are perfectly jaded to stripping down and re-garbing in front of strangers, and will "pffffft" at the offer of you leaving the room, it's still polite to (at the bare minimum) turn your back until she's ready to go. Don't watch or stare.
6. Ask about her personal/sex life.
I mean, really now? There's no reason why that information would be necessary on a shoot. Same thing with asking if the carpet matches the drapes when you're shooting a fully-clothed set (possible exception: pre-shoot discussions about doing nude work). This isn't making conversation, it's making a creeped-out model.
Are there any exceptions to these boundaries? Sometimes. A good rule of thumb is follow the model's lead. She's not going to bring up a subject she's uncomfortable with on a lark, so if she veers the conversation into one of the above territories, it's probably OK to assume she's not going to suddenly morph into SuperPrude and, if you're comfortable, you can continue in that vein. This is a social skill called "mirroring". Everyone does it, it's how rapport is established. Again though, make sure it is YOU mirroring HER, and not the other way around, 'lest the model is of a straighter-laced character and subsequently squicked. Word does get around about unsavoury characters, so do your best to be aware of how you come off and keep your manner professional.