Theories on the Willow Incident

[Minerva the Bookwyrm]

An incident in the Potterverse that has invariably fascinated me is the one where Sirius tells Snape how to get into the Whomping Willow tree passage. Then, James risks his own life to save Snape.

Some things we already know:

Sirius would not have felt guilty if Snape died judging by what he says in the Shack in PoA.

Snape suspected Remus was a werewolf BEFORE the Willow incident judging by his memory in which he was speaking with Lily about it. JKR said The Incident happened in their 6th yer, and the conversation with Lily occurred before the "Mudblood" confrontation in 5th year.

Questions this raises:

What was Sirius' motive for telling Snape?

Sirius never felt guilty about almost killing Snape, but did he feel guilty about betraying Remus?

Why did Snape go there when he already suspected the danger that awaited him?

What was James' motive for rescuing Snape?

Do you think James knew Sirius had told Snape before Snape went in, and he just assumed that "Snivellus" wouldn't enter, or that he didn't know what transpired until Snape had already gone to the Willow?  

[Minerva the Bookwyrm]

What was Sirius' motive for telling Snape?

I've never thought of a satisfactory answer to this. In some fan fics it's because Snape "went too far" and Sirius believes he deserves to die for it. In other fics it's because he's angry at Remus. I don't know...Loyalty is so significant to Sirius (Remember how he treated Wormtail for treachery?) so telling Snape seems so out-of-character for him that I cannot conceive any reason why he would do that.

Sirius never felt guilty about almost killing Snape, but did he feel guilty about betraying Remus?

I hope so. He nevers says so in the Shack in PoA, but he probably already apologised to Remus a schoolboys, and at that time they had more important things to worry about. Of course, maybe because he was only trying to hurt Snape (which he doesn't regret), not attempting to hurt Remus in anyway, it's posible that Sirius' mind he has nothing to be sorry about since harming his friend was never his intention.

Why did Snape go there when he already suspected the danger that awaited him?

One of two reasons, I'm sure:
1: He presumed that the secret he'd find out was unrelated to Remus and Black had told him in order to anger his friends since Black was upset with them.
2: Snape suspected that he would be attacked (maybe by Potter and Black rather than a werewolf), but knew that if he let the ambush happen and later reported it then he could get Potter and Black into trouble.

What was James' motive for rescuing Snape?

In my opinion there is no doubt (as I expressed in a fan fiction I wrote A Very Sirius Full Moon) that James' motive for saving Snape was to protect Remus, not Snape. Snape could have injured Remus with spells in order to protect himself. Also, if Remus killed Snape he would be expelled at best, and executed by the Ministry as "dangerous creature" at worst.

Do you think James knew Sirius had told Snape before Snape went in, and he just assumed that "Snivellus" wouldn't enter, or that he didn't know what transpired until Snape had already gone to the Willow?

Personally, I don't think Sirius told James until it was almos too late, but either one seems plausible to me.  

[SuzelovesJamesPotter]

What was Sirius' motive for telling Snape?
I think that James' and Snape's rivalry was what start everything. Sirius was immature and must of thought it'd be a good prank. I don't think he realized how hard it would be on Moony. I think he was just trying to prank Snape and he got this 'marvelous idea'.. not.
Sirius never felt guilty about almost killing Snape, but did he feel guilty about betraying Remus?
Of course. I think he didn't mean to hurt Remus. When they got into their older years and Sirius had broke out of Azkaban, he already lost James and Peter was a traitor. Remus was his last friend alive and I'm sure he felt remorse realizing what he did. After he knew Sirius didn't betray James and Lily, Remus stuck with Sirius until his death. I am sure Sirius felt horribly guilty for doing that to Remus even if it was a mistake.
Why did Snape go there when he already suspected the danger that awaited him?
I think he wanted to prove to Lily he was right and gain her friendship back. Also, He wanted everyone to think badly about the maruaders and this was a way to get them that way.
What was James' motive for rescuing Snape?
James I think is a really a good person. That was the main reason to me. Plus, I'm sure he was trying to save his friend's butts. Remus would've commited murder if James didn't stop him. That's Remus' worst fears. James' friends were obviously very important to him. Lastly, what would Lily think if Snape had gotten killed and James was there and didn't save him? James loved Lily. End of story.
Do you think James knew Sirius had told Snape before Snape went in, and he just assumed that "Snivellus" wouldn't enter, or that he didn't know what transpired until Snape had already gone to the Willow?
No. I think James would've stopped him. Or maybe he tried. Either way, James wouldn't of let it happen if he could stop it.

There is my thoughts. I hope I got everything right...