Paris - When it Sizzles (1964)
Playboy screenwriter Richard Benson (played by William Holden) has been paid to write a screenplay for his boss. In his arrogance he's squandered his time in Paris drinking, tanning, and doing just about anything but writing a screenplay. Two days before the script is due he hires Gabrielle Simpson (the adorable Audrey Hepburn) to type the script. Richard and Gabrielle come up with various off-the-wall scenarios for his screenplay, called The Girl Who Stole the Eiffel Tower. As the screenplay they write slowly develops, romance blooms between them.
This movie was filmed a decade after Hepburn and Holden's affair (during the filming of Sabrina) and apparently Holden wanted to pick things up where they left off. However, there is still visible chemistry between them.
The movie was later remade as Alex & Emma, starring Luke Wilson and Kate Hudson.
Here's the TCM trailer: