ApolloImperium

lawlz, a man ALWAYS knows when he's "getting hard." a lotta girls may not even have the slightest idea about this, but there's actually a long, long, LONG time where the guy's not hard at all (depending on the situation of course).
assuming all things equal (no pressing urgency, etc), a guy sorta starts getting "tingly" let's say. it's a shift in his breathing, in the flow of his blood. usually, he'll do a sorta half-shift, a reposition of sorts, because his dick is getting "twitchy" and he doesn't want it to show through his pants. any man who's finished puberty is EXCELLENT at controlling and concealing his erections. it's sort of a "trial by fire" if you will. after a few years of suffering from rock-hard erections that come and go in a flash, at incredibly inoppertune times, we kinda learn to deal.
but more to your original question, the best analogy I've ever been able to come up with is this: all men have a penis. a penis he likes very, very much. easiest explanation? girls, think about your tongue. think about how often you move it around in your mouth, outside of talking or eating. think about how sensitive it is, how often you accidentally burn or bite it. how generally aware of it you are at all times.
now put that between your legs. that is a penis. men are ALWAYS generally aware of their penis. it's a protruding object in a very inconvenient place. god, I can even think of the number of times I accidentally sit on my balls and flinch in pain every day. hah, well it's probably not even one or two times a week, but still, those few times I do I remember it (ow!).
when I stand up, I feel it drop and shift. if I switch positions, sometimes I have to adjust it because otherwise it will look REALLY FREAKING OBVIOUS where it is. I am aware of it at all times. I am on constant alert for flying/falling object that seek to harm it, because GOD does that hurt. it's like getting knifed in the kidneys, even with just a glancing blow. all men know this, all men fear this (well, maybe not the weird, creepy guys who like to pay hot women to knee them in the balls, but you're probably not writing about those in slash are you? ).
anyways, does that kind of give you some background information about the penis itself? the whole tongue-thing should sort of be kept in mind when trying to explain an erection. I don't know how in-depth of an explanation you want though.
I mean, it's really kind of two-part system. the guy started feeling aroused first. like I mentioned earlier, it's just... a shift in the blood. his lips part maybe, or his interest increases. it's very subtle. basically though, his body's just being like "hey! hey dude! if we're gonna get hard, I'm ready! just lettin' you know!"
his dick's not even hard at all at this point. it probably hasn't even swelled, to the naked eye. it's kind of... poised, though. on the precipice. then, depending on whether there's going to be more sexually stimulating stuff going on, it might get harder, it might soften, depending on how anxious he feels about it. like, if he's kinda into what's going on, it'll slowly inflate as he gets more into it. if he's turned on, but he knows he's not getting any or it's not "stimulating" enough, or if he simply has a great deal of self-control, he'll stay in a semi-hard state for a bit before his body realizes nothing fun is gonna happen and gives up. he may be still *aroused* for a good while, but his erection is not into it. his body has better things to do (and, of course that doesn't preclude him from getting hard later).
physically, there's ALWAYS readjustment. depending on the position his penis is in, getting hard could be a fucking *bitch*. if like, the penis is tucked downward, alongside the crease of the hip and leg, getting hard is fine. pleasant, even! the resistence gives a bit of friction with each pulse of blood and the guy's kind of "safe" because his dick's tucked away in an unnoticable spot. if it's along the length of the leg though, fuck! that shit sucks! you either have this obvious tube arcing over one leg, or it starts to get REALLY hard and slides up to nuzzle your waistband! then the guy has to shift position, or readjust somehow to either relieve the tightness of his stomach pushing against his dick pushing against his waistband, or to conceal the obvious boner he has.
and then for the actual penis, that really depends on the guy. some dudes have a high pain tolerance, and can jerk off with just their hand. others just require spit, and then some guys (usually uncut) won't let you even think about touching their dick without a handful of lotion or lather. uncut is different too, because it's like a tight ring of skin sliding out over a very sensetive tip. purse your lips tightly and then slide your tongue through them. it's kind of like that, depending on the tightness of the guy's foreskin (ranging from hella tight to really loose).
then, depending on how horny the guy is, his dick could be a mere shadow of its real self (if he's not interested, anxious, bored) or it could pack on exxxxtra inches like no tomorrow, surprising even the dude who owns it (if he's SUPER-ULTRA-MEGA turned on!).
of course, neither case is necessarily a garauntee of climax. erections can be so hard the dude's willing to drive it through a brick if he has to, and then other times he's going great and suddenly he feels the blood returning to the rest of his body and he's like "no no no stay hard please stay hard!" only now he's super worried and the hot babe looking at him is making him feel emasculated and weak and that's only making it worse and now his erection has turned into a limp droop
but short answer is: a man might be surprised by his erection on the rare occasion that something has captured 99% (NINETY NINE PERCENT) of his attention/is so unbelievably erotic that his brain is on overload and he can't hardly think, but generally no. guys know when they're going to get one, whether to stop it from happening, and how to shift, adjust, and sit so that no one around them has a clue.

Hey Wolfy your going to Die...In My Pants

http://www.youtube.com/watch?v=bRGh7lvsHsA&feature=sub
be sure to look under your bed.