I always appreciate it when a film that loosely models itself on the familiar structures of the rom-com or family melodrama genres. Convention suggests that there should be a scene where someone decides to embarrass or humiliate Lars by pointing out that he's crazy/perverted for thinking that Bianca, his real doll, is a real human being, and Lars should deny that it's true, but suffer a breakdown of some sort because he knows that it is. (Similar to any one of those movies premised on the main character having a secret that is revealed by the antagonist before the protagonist can reveal it/come to terms with it himself.) Thankfully, Lars and the Real Girl teases at this potential swerve on very briefly - and in a totally convincing way - and never revisits the cliché again.
The film also has an atypical take on Lars' "delusions", though I'm less certain of how that take makes me feel. While it's suggested that Lars' creation of Bianca is connected to his father's recent death and his sister-in-law's pregnancy, there is no simple and obvious causation - the moments in which his delusion start and stop don't correspond in any direct fashion. It's also never presented as a pathological condition, aside from it being labeled a delusion; it's simply how Lars' mind has chosen to deal with its depression. Conversely, I'm sure that it'll earn the ire of certain mental health profession: Lars' delusions are presented as something that can be overcome with love and patience, and the condition is one that Lars himself can overcome if he so chooses. It's not that simple, of course, but it appears impossible to undertake the one - presenting the delusions as part of a normal process of coping - without eliding the other - that is, medicalizing and medicating the ostensible illness. But this is a debate for people who are far more versed in the nuances of the discussion to undertake.