Billy to Mackenzie

(At the prom)
Mack: What are you thinking?
Billy: I'm thinking how beautiful you are... and how long it took me to realize it. No, I always knew it..."

(At the hideout)
Billy: You know how before you had a backup plan, and I told you to forget about it? Well I've thought about it, and I changed my mind.
Mack: What are you saying?
Billy: I'm saying that if you want to run away, I'm not gonna stop you... on one conditon... I get to come with you.

(At the Abbott house)
Billy: Tell me I'm not crazy. Say that you feel it too.
Mack: But it's wrong. It shouldn't be happening. Half the time I can't even stand you.
Billy: However crazy it is, these feelings are real.

(At the Chancellor estate, Mack's bedroom window)
Mack realizez that Billy is at her window.
Billy: I came to see you... after what happened tonight. I know you felt the same way I did, on the dance floor. (he takes her hand and kisses her)

Between The Gang

(At school)
J.T. (referring to post-prom parties) clearly explains to Raul that Billy isn't welcome.
Raul: It's just as well. Billy doesn't need to be around alcohol, let alone jerks like you!

Billy to Jack/Ashley

(At the Abbott house)
Ashley is crying
Billy: What's wrong, Ash?
Ashley: I was just thinking... about you and Mack at the prom.
Billy: (jokingly) That's what's got you so upset?
Ashley: (laughs) No.
Billy: Then what?
Ashley: I was just thinking about how, when you're young, love is so pure, sweet and simple.
Billy: Is this about the conversation we had earlier (referring  to Ashley trying to get him to 'fess up about Mack)...
Ashley: I'm just saying, Billy. When you get older, that's when it gets complicated.
Billy: Tell me about it.
Ashley: But it doesn't have to be, Billy. Atleast, not yet.
(Ashley leaves and Billy picks up the phone to call Mack, but then decides not to)