Jeśli ktoś się głowi nad interpretacją to podaję bardzo fajną (zapewne nie jedyną) interpretację z IMDB:

"nobody seems to understand enemy, I loved the movie personally and you understand it if you watch it
twice and pay attention.

okay ill try to explain it, it is about one man fighting with himself , while it is portrayed as two people
(Anthony/Daniel and Adam) only one of them is real, and its Adam, I know this because in the
beginning on the phone his mother is speaking and says "Adam honey" and also later on in the movie
his mother also tells Adam (who is not an actor) to give up his dreams of being a third rate actor. now
the story of this movie is hard to understand , its about Adam and Helens marriage and the remaining
spark of an old affair, in the beginning you see it open with Jake Gyllenhaal in what is presumably a
strip club, eventually you see a woman come out and step on a spider, the woman is Mary, Adam's
affair , after that it opens to Adam sitting in his car as you hear his mother leave a voice mail for him.
Throughout the movie you see spiders, spiders represent his affair with Mary, and how he still thinks
about it, and when you see Adam with Mary he really isn't its just representing his still lasting feelings
for her. you see when Adam calls Anthony it really wasn't Adam, it never happened, the whole phone
call scene was his imagination, helping him realize he needs to forget about Mary. I'm gonna skip a
bunch of unimportant things and try to go as chronologically as I can. At the point where Anthony tells
Adam hes going to have sex with his girlfriend , that's really just Adam thinking to himself about him
needing to get rid of Anthony and the memories of Mary. so when Anthony goes and takes Mary out
that doesn't really happen. whats happening is Adam is thinking that up to help in the getting rid of
memories, it is also revealed that Adam is the real person when Adam and Helen lay down and Helen
asks how was school . the scene of Mary and Anthony having sex and Mary freaking out is Adam
realizing he is married and needs to get rid of the memories, when Anthony and Mary crash the car it is
Adam killing off the memories , squashing the spider per Se , this is supported when it zooms in on the
crashed cars window the crack looks like a spiderweb, when Adam wakes up the radio speaks of a
car crash but it says no details so It is likely that was just chance. also Helen reminds him about his
mother calling him, which you should remember from the beginning of the movie. when Adam opens
the letter and gets the key, it is a key to the strip club place, and he says to Helen he might have to go
somewhere tonight. he goes to look at her and she is a huge spider, this represents how him going to
the strip club would bring back the spider. during the movie the spider gets gradually bigger until it is
huge and he cannot get it off his mind. so I hope I've explained well enough to help you understand
what an amazing movie this truly is."