Trivia: The film's director, David Cronenberg, has a cameo in this film as the doctor who helps Ronnie deliver her maggot baby.
Trivia: Michael Keaton was the first choice for Seth Brundle.
Trivia: Tim Burton was offered the chance to direct.
Trivia: At least 20 different versions of the 'space bug' (Brundle's final mutated form) were produced, many of them bearing a mostly symmetrical design so as to facilitate the building and operating of animatronics and puppets. One by one they were duly turned down until the special effects department finally in frustration came up with a totally unfeasible asymmetrical look that would take months to construct and be an absolute nightmare to work on set. The producers loved it from the first sight and that's what appears in the final film.
Trivia: An extended ending showed Geena Davis dream about giving birth to a baby with butterfly wings. This was cut from the film to emphasize the drama of Seth's death.
Trivia: Two additional deaths for Brundle were proposed in addition to the filmed version where Veronica shoots him with the shotgun. One involved him trying to attack her one last time, only for Stathis to grab the power cable seen trailing from Brundle's telepod-fused body and shove it into a nearby outlet, thereby electrocuting the creature. The other involved Brundle crawling towards Veronica and then expiring from his injuries, sparing her from having to kill him.
Trivia: Originally the steak experiment scene had one additional line of dialogue from Ronnie in which she replied to Seth's declaration of teaching his computer about flesh by quipping "What are you going to do? Read it 'Naked Lunch'?" Cronenberg intended for this to be a little nod to the fact that he was planning to do a film adaptation of 'Naked Lunch' but eventually decided it was too allusive.
Answer: Brundle's just had all of his atoms separated and then joined back together with a massive influx of electrical energy; certainly that would generate some sort of sensation in cells that have never experienced it before. No reason to believe it couldn't manifest as euphoria and strength. As far as the transformation: Brundle's cells have been put back together with fly DNA in them, but with very little actual fly material, initially. It's reasonable to expect that as his cells die off and regenerate in the usual ways, they are replaced not by human ones, but by human/fly hybrids. As this happens, he becomes gradually more fly-like.
Rooster of Doom