Directing a bar dialogue scene

What happens in the scene? How do the characters feel about each other? What is the atmosphere in the bar? How does this scene fit into the rest of the film?

Two possible examples…

Perhaps it's a very intimate scene where they finally confess their love for each other. The camera gradually moves in cut by cut to tight close-ups, allowing the audience to pick up on their facial expressions - especially eyes and lips. The bar is lively and throbbing, which only heightens the sense of closeness between the two characters as you emphasise the disconnect between them and their surroundings with longer lenses.

Alternatively - it's a business meeting between two people who just don't get along; neither of them wants to be there. The hotel bar in the early morning is drab and dull. The camera mostly only shows one character at a time to highlight the distance between them. If you have to show both together, consider using a wider lens to exaggerate the physical distance.

Two very different scenarios there, neither of which may be remotely right for your film, but I hope that gives you some food for thought.

(Also: now's a good time to start thinking about sound, while you're still in the early stages of planning. I don't feel at all qualified to talk about this, but hop on over to AudioPostExpert's thread The Principles of Sound Design to see if you can get a feel for the sonic as well as the visual.)

The delivery of the dialog must be thought out completely and thoroughly conveyed to your talent before you begin shooting.

As Chili mentioned the type of bar and the time of day (I'll leave the motivations of the characters to others) will make a huge difference. The more motion/action that is present in the scene means that the actors will have to increase the volume of their voices/delivery. Nothing annoys me more than the scene taking place in a club with dozens or even hundreds of people, the DJ or band pounding away, the crowd partying hard, and yet the characters converse as if the they were having an intimate moment in their living room. The audience never thinks about this consciously, but they will notice that something is wrong, and that will pull them out of the moment, and that works against you as a storyteller.

Once you have specifically defined the scene I can give you a few more tidbits on capturing the dialog and prepping the set and cast.

I think a great example of Alcoves point (as to how it SHOULD be done) is the bar scene near the beginning of the first matrix movie. Where Trinity is leaning and talking in Neos ear, I love that scene.

Another terrific example is the club scene in "The Social Network." All of the characters are speaking with raised voices. The sound-for-picture community (half) joked that the film should have received the best sound editing and sound mixing Oscars on the strength of that scene alone.

The Social Network has masterful sound design, though it's really very invisible. There's a featurette on the Blu Ray about sound design for the movie, and is a great watch for anyone interested in Sound Design