The big kiss in Chan Wook Park's upcoming I'm A Cyborg, But That's Okay was apparently never scripted. In fact, the director didn't intend for such a scene at all - but the actors did. Once they 'improvised' the lip-lock, (in a kind of cold-blooded, pre-meditated improvisation, if there could ever be such a thing), Park was convinced and egged them on to really go for it.
In the finished film, the kiss is worked into the plot and character arcs brilliantly - our heroine, who believes herself to be a cyborg, doesn't admit any romantic element to the clinch, telling herself that the kiss is just her means to recharge batteries.
Ahhhh. How sweet. Any kiss-based insurrection like that on one of my sets and I'd go round spiking the lipgloss.
Or maybe not.