What Exactly is Romeo's Plan?

While answering questions over at the new place, I found an angle on the big fight scene in Act 3. What, exactly, do you think is Romeo's plan? I realize that he wants to prevent the fight - but more specifically, is he trying to keep Mercutio from hurting Tybalt? Tybalt from hurting Mercutio? Does he even think that far?

It's probably unanswerable, but that's never stopped us. I think it makes for an interesting take on the character, because if he thinks "I need to hold back Mercutio before he kills Tybalt," well then he's basically just sold out his best friend, hasn't he? It could be, of course, that Romeo simply went for the logical person - Tybalt was trying to kill *him* (Romeo) after all, and if Romeo suddenly stepped in from of Tybalt's sword, that would likely not have ended well.

While we're on the subject, can we talk about exactly what Romeo's mistake is, here? I've always sort of thought of the big moment as "Romeo grabbed Mercutio." But why, is the question. Romeo appears to walk into that fateful encounter thinking "I no longer see the Capulets as my enemy, therefore the Capulets are no threat to me." That's a big lapse in character judgement. Tybalt has never been a threat because he's a Capulet. He's a threat because he's a bad guy. This, ironically, is something that Mercutio knew all along. Mercutio didn't hate Tybalt because Tybalt was a Capulet. Mercutio hated Tybalt because he *is* a good judge of character, and knew that Tybalt was trouble.