A typical farrier visit is pretty straightforward- a lot of farriers aren't big talkers, so unless you ask questions a lot of times they'll just do their job and keep their opinions to themselves. Usually they'll let you know if there's a major problem, but will often keep minor problems to themselves. I always like to ask if they see any issues coming up (thrush, new flares, etc.) and if there's anything they'd recommend I do for it. For a new horse or farrier, I also ask what they think of the hoof shape in general, what flaws they see, and what the plan is to correct it.
I always make sure my horse is ready at the appointment time, with any mud washed off of the lower leg (and dried as much as possible), hooves picked out, and any major dirt brushed off the body. Have fly spray handy if needed.
As far as giving treats, I think it depends on the horse.
My horse knows what's expected of him with the farrier and isn't at all scared- if he starts pulling his legs away from the farrier or tries leaning on her, then he gets reprimanded. I'm not big on treats, and he's not really all that big on them either, so I don't generally have treats on hand to give him afterwards (really, for him, being done is enough reward!)
Then there's the pony mare I've been working with for the past few months. She's nervous and fearful and used to have to be sedated for the farrier. She's also highly food motivated. Treats were an important tool in getting her to stand for the farrier and reinforcing it as a positive experience. She doesn't get sedated anymore, and the farrier thinks I'm a horse whisperer