That's her story
I'm surprised at most of the comments on this board regarding Laura's and Alec's relationship. Everyone discusses it as if it all happened exactly as Laura presents it.
The story is told from Laura's perspective. But she is not a reliable narrator. For example, her voiceover begins by telling us she's happily married. She tells us that she forgot all about the kind stranger who removed the grit from her eye until she met him again. She tells us that even then she doesn't think that much about him. All the while it's very obvious none of that is true. The truth is plain to see on the screen: she was attracted to him from the start. Look at her reactions to him, that's not an indifferent woman, no matter what she's pretending.
Bearing this in mind, I think one has to reconsider a few other things. Naturally, Alec would appear as the pursuer. It would be him who first talks of love. On the other hand, it would be Laura who appears as the scrupulous one, who voices(!) all her doubts, leaving Alec nothing much to add but: "That's how it is for me, too." That is exactly how one experiences and remembers events, subjectively, and makes them fit one's perception of one's own character. The reaction of Alec's friend who lent him his flat, which she can't have witnessed, is supplied by her imagination - it must have been this shameful - because she feels so much shame. Finally, it seems her husband doesn't notice much because she doesn't pay much attention to him while she's constantly thinking about Alec, and that only changes at the end.
Of course, every viewer is entitled to their own take on the characters of Laura, Alec and Fred. I do think though that it helps to keep in mind that what we have is just Laura's point of view.
clowns to the left of me, jokers to the right, here I am...