Which 1986 film had the tagline "Be afraid. Be very afraid."?

The phrase 'Be afraid, be very afraid' is a warning that something dangerous is imminent. In reality, this is usually said with comic intent. The thing being warned of is more likely to be mildly unwelcome than actually dangerous.

This phrase originated in the 1986 horror film 'The Fly', written by the Canadian David Cronenberg and starring Jeff Goldblum (as Seth Brundle) and Geena Davis (as Veronica Quaife). The shortened expression 'be very afraid' was already in use in the USA prior to 1986; for example, it was used in the television series 'All My Children' in 1970.

The plot of 'The Fly' has Brundle as a scientist experimenting with teleportation. The scientist is brilliant but eccentric and, naturally, the casting director thought of Goldblum. Just as naturally, before we get far into the film, the experiments begin to go wrong.

Quaife is a reporter working on the teleportation story. When it becomes clear that Brundle is starting to turn into an insect, he pleads with one of the characters "don't be afraid" and Quaife's response is: "No. Be afraid. Be very afraid."