What about Daniel? ** Spoilers **
I found Daniel a potentially interesting antagonist, yet his part in the film was so sketchy I wasn't sure if he was stalking the Real or Fake Lisa.
Had Alex tricked him also into thinking she was Lisa? Or was his note to the Real Lisa. If so, she had given him the key to her place, and trusted him to give it back w/o making a copy when she dumped him.
Plus, the question of his wife's death. Was it staged? Is that what scared Lisa off? Or was Lisa a commitment-phobe, frightened off by the possibility that Daniel was now free to be with her fully?
If he was a potentially murderous character, why not at least hint that he was still following (real or fake) Lisa, and would mean future trouble for Luke, Matt or "next guy in line?" That would've added some spice to the movie for me. Christopher Cousins is a fine actor, and proved to have a dangerous edge in "Breaking Bad." He could've added a lot to the movie!
But, in this movie, we know less about Daniel than any other major character. Except, maybe Lisa!
I thought this was basically a chick-flick: all about relationships and communications, with no real dangers, and the protagonist probably learning nothing (based on his hasty, emotionless dump of Rebecca at the airport).
Thus maybe a good tagline would be: "Women good, Men bad!"