I remember once discussing the Fall of Adam and Eve with my boss's wife (because who doesn't talk about such things with their boss's wife?), and she posed a question I'd never thought of that really struck me:
"When the serpent tempted Eve, where was Adam? He should have been there to protect her."
Wow. That's a great question. (Which, as my cousin Joe has pointed out, is a euphemism that means, "I don't know.")
Just where was Adam? Tending the garden? Picking (other) fruit? Milking the cows? Did he know there was anything dangerous in the garden that he might need to be on the lookout for? Would he have left his wife by herself if he'd known there were cunning talking serpents slithering around the place?
What I most appreciated about her question, though, was the assumption that it was Adam's duty to protect his wife from harm. Not that Eve was too weak or dim or otherwise incapable of looking after herself, but simply meaning that Adam had a responsibility to look out for her. St. Paul says that a man should love his wife as Christ loves the Church: he should be willing to give his life for her. Adam should have been willing to take that snake bite rather than let his wife come to harm.
My boss posed a question of his own: why did Adam eat the fruit when Eve gave it to him, when he knew God had told him not to? And my boss had a theory which I found moving: when Adam saw that Eve had eaten the fruit, and knew she was going to be in trouble, he ate it, too, out of solidarity, so that whatever happened, they'd face it together.
I'm not sure if this is the proper answer, but there's something beautiful in it: that Adam was so bound to his wife he would face God's judgment with her. Not that we should necessarily follow others into sin, but there is a sound principle there of wanting to be with the beloved other where they are in their time of trouble.
What would have happened if Eve had eaten the fruit but not Adam? How would that affect the transmission of original sin? St. Paul contrasts Christ's obedience and Adam's disobedience; well, what if Adam hadn't been disobedient, only Eve? Or what if Adam had been the one to eat the fruit but not Eve? How would things have been different if there'd be an "obedience gap" between our first parents? Would only the one have been punished and died? Would God have created a new spouse for the other and started the human race over again, free from the stains of its past members?
I have absolutely no idea what the answers are to these questions. But they're interesting to think about.