Blow Out is one of the few films that really is about voyeurism. Revolving loosely around a search for a perfect scream, De Palma's best film is related to the paranoid conspiracy films of the 1970s like The Parallax View and All the President's Men - and most obviously an homage of sort to Antonioni's masterpiece Blow Up - but is ultimately an exploration of an artist's relationship with his or her work, especially as it relates to cinema. That is, Blow Out is a movie about the unique experiences of making movies and consuming them.
John Travolta plays a sound man working on sleazy exploitation horror film when he is inadvertently thrown headfirst into a conspiracy that has left a governor and prospective candidate for president dead. The steps Travolta takes to develop evidence that proves the governor was murdered rather than killed in an unfortunate accident grow increasingly desperate throughout the film, culminating with the highly emotionally charged climax which takes place during a massive patriotic (but fictional) Philadelphia street parade.
Travolta is constantly struggling with the differences between observer and participant in the film. Before the car crashes, he listens in on a couple's romantic conversation. In the flashback to his days going after corrupt police, Travolta listens from afar, never sure when to join in. It is clear Travolta's character takes definite stances on this element, which makes his struggle to save Nancy Allen in the final moments that much more compelling. His desensitized response to the horror sequence at the beginning of the film emphasizes his own inability to connect with the outside world (much like James Woods's character a few years later in Videodrome) but his use of Allen's scream in the final moments seems to demonstrate that he is forever changed, finally sure he can always twist reality to suit whichever fantasy he chooses.
Blow Out becomes in those final moments one of those eternal fun house mirrors, a movie about movies. We are horrified by Travolta's choice, but deep down we know just how he feels - we were, after all, just watching a movie that featured Allen's cries and it was indeed a good scream. It's also, however, born out of the dying embers of the 70s, when the mistrust in authority gave way to cynical opportunism once the defiant realized they were powerless. Travolta's ultimate surrender is from integrity to commercialism, and the purity contained in Allen's last gasp is twisted into exploitation. The insistent, defiant hero has sold out and joined the firm.