It was a thankless character to play. Laura's the mature one, ready to settle down. She already gone through her own midlife crisis by the time the movie opens it's why she broke up with Rob.
As a result for most of the movie she just sits around and waits for Rob to grow up. She makes interesting faces every now and then, but it's a pretty one-dimensional part.

Don't listen to these bums, Iben. You were great.

The problem at the start is that Rob thinks his relationship has become wooden and dull, and is seeking an exciting fantasy relationship.
As such, Laura seeming wooden and dull is a good thing.

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Upon subsequent viewings I agree. Her performance was very wooden and dull.

I can understand why you feel that way. He performance wasn't terrible but I do think she was miscast. I still love this film so much.
I think the point of casting a little know actress who is pretty but not a super model was to emphasize the films theme of maturity. Laura is a very good girlfriend but Rob wants the fantasy girlfriend which Laura isn't.
The Laura character is better developed in the novel. If you haven't read it I highly recommend it.