The introduction of the various characters tell us a whole lot about how they will be for the rest of the film. And, I don't just mean the fact that Sam (Hayden Cristensen) gets out of bed, huffs and puts his neck in a makeshift noose hanging in his closet. Or that George (Kevin Kline) goes outside to pee, facing the ocean, first thing in the morning. I'm talking about clothing. George is in his underwear, white briefs. He's old fashioned, but he also has no shame. Sam wears boxers and a t-shirt, both dark. (They will both also be naked later in the film.)
Alyssa (Jena Malone) is also introduced in her underwear (and will be naked later), leaning out the window to watch George. Her mother Colleen (Mary Steenburgen)--who seems to always be home even though she's a single parent--is introduced in pajamas (and will be naked later). Robin (Kristin Scott Thomas), on the other hand, is introduced already fully dressed and ready for her day (even though she, also, stays home all day). George has to dress for work, but Peter (Jamey Sheridan) is already dressed up when we first see him.
I imagine it's simple. The characters introduced in underclothes or bedclothes are more open as the story goes. The characters introduced fully dressed are not. The exceptions--and the dichotomy of them is interesting--are Colleen and Robin. Robin spends the second act opening up to George, her ex-husband, ultimately at the expense of her relationship with Peter, her current husband. And her clothes loosen as the film goes. Colleen remains mostly separated from the main plot, and never directly participates in the housebuilding, but she does go from complaining about George being outside in his underwear to hiring workers to help finish the house.
Colleen also has a scene in which she is wearing for the first time a brand new matching set of bra and underwear, still with the tags on. After getting together with Josh (Ian Somerholder), she, understandably, feels sexier. So she dresses the part.
Peter, after he has left Robin (or, really, after she has (emotionally) left him), finally gets to be in a scene without a dress shirt on...
(Actually, we see him in black pajamas before that.)
And, I'm rambling.
There is a point here, but it's an obvious one. Clothing makes an impression on how people see you. How people see you makes an impression on how you act in response. You become the clothes you wear. Sam removes his labret piercing (the one below his lip, in case you don't know) and his earrings, he stops wearing makeup and, though his hair is still partly blue, it stands out a little less from the black as the film goes on. It's a simple trick; change the costume, change the makeup, and we can see the changes. Internal becomes external. Robin wears a ponytail or a sweatshirt, and we know she's not as tightly wound as before. (She also prepares dinner, when there was a housekeeper earlier in the film. She's becoming a mother again.)