I agree with David. It's always bast to shoot these kind of stuff, but I know it's easy to say that when you're not the one that has to do it, so here are some ideas.
It's not ready-to-use recipe but it will hopefully give you and others something to think of.
Aside of the whole real-look problem (displacements, highlights, reflections and stuff) the basic problem is animating the water drops. My first try would be using some particle engine - in this case Particle Illusion instead of what you probably thought I'm going to say (Trapcode Particular). The reason I choose Particle Illusion is that it has more to do with simulations, than it has with generating cool looking particles.
I would combine Particle Illusion and Advanced Lightning to create a path that the water drops will take while pouring from the top of the window. That would hopefully create an animation of particles traveling on the lightning.
Then I'd apply some blurring filters, threshold and others to create the drop shape. Then some displacement, turbulence displacement, and finally CC Glass and Mr Mercury for final touch-ups...
I haven't tried it, so it's hard to say what this technique is missing, but I have a strong feeling it has something good to it and that it's worth a try.
Good luck.
Maltaannon
maltaannon.com - Free After Effects Video Tutorials and more
