Hmm, I may be helped then by my complete ignorance of all her other work, on screen and off. I didn't have that hurdle to get over, buying her in this role, since this was my first exposure to her afaik.
I have seen this! Here's my complete memory of it.
Smart Streisand professor, eating junk food to explain frumpy.
Montage of exercise, hair and clothes
Vibrant Streisand full of life and super confident.
That is sort of accurate. See if any of this refreshes your memory. It is just my interpretation though.
I don't feel like her confident personality changes that much before and after the montage. She was always self-aware, and confident enough to be self-deprecating even in front of her class. Her frump had no effect on it. She just wasn't very motivated by the male prospects in her life to go to all the effort of looking nice. However she happens to be exactly what Bridges is looking for at the moment he finds her. All the exercise and clothes and hair is tactic to try and make him have sex with her, since she ultimately comes to the realization that she's not willing to settle for the platonic relationship that she initially agreed to.
The premise itself was, I'm sure, written as an absurdity... something they felt the audience would laugh at. "As if guy would NOT want sex, hahahahaha" kind of thing. I actually didn't find it all that absurd though. Maybe I've been listening to too much Dan Savage or something where they have regular discussion around asexuality, low libidos, companionate relationships and that sort of thing, but I just took Bridges' character at his word. It was an intellectual decision not to let sex into a relationship because he wanted stability and saw sex as destabilizing. Fair enough I thought. You do you. Streisand, with her lack of good prospects, is willing to be with him because he's nice, intelligent, and attractive. The best thing to come across her plate in some time. The no sex thing is, initially, a price of admission she's willing to pay.
I forgot what point I was trying to make... I'm just describing the movie now.
Blah blah blah, maybe the premise has actually improved with age.