Theatre in a pub where the audience can sit and watch the show or talk among themselves. Like you would see a band in a pub, the performance would be set up in much the same way.
This would, as you can imagine, create so many problems within the context of a play that a band would not suffer. Firstly, you need to grab the audience's attention. You are competing with the attraction of idle chatter, so you need to have something that will make the audience watch you rather than talk to the person next to them.
You also need to be able to follow it from any point in the narrative and then be able to follow it all the way to the end. Thus, the characters need to be very clear and relations need to be constantly reaffirmed. If someone comes in half way through and cannot follow the show then they will not watch it, simple as.
The thought of Ambient theatre is something that you can dip in and out of it without too much worry and that it can also go on in the background. This would mean that the subject matter can't really be too taxing, complex or challenging as that would need a greater level of concentration than a pub audience would be willing to give.
If it turns out that an audience would stop to listen and watch the performance (which I wouldn't think would be the case, as such) then you could probably push the boat out a little and try and engage the audience a little more, but if this continues to be ambient in nature and response then you would need to weave the context and subplots into the piece through subdues and lighthearted theatre.
The style that it would take makes me think of Brecht in manner, as you would have limited set and props and need to make up for this in other ways. Brecht tried to reach an audience through means similar to that which i would need to use. Devices such as use of signs, explanations of scenes in prologues and epilogues, direct audience contact, stepping in and out of character would be employed to try and reach the audience.
Another obstacle that would need to be addressed in this sort of performance would be the role of the drunk. As with watching a band or a stand up comedian, you would probably get interruptions from people who are drunk or just think that you are wasting every bodies time. This, you would have to deal with, either as the character or by stepping out of character and confronting them directly, and it would really depend on the situation how you would address this.
May people think that the theatre is not accessable enough and as a competition for TV, cinema and the pub, the theatre is losing out on reaching a mass audience, especially out of London. The way to combat this, is to take the threatre to them. Don't just wait for the public to come to you because they won't.
"Boring an audience is the one true sin in theatre. We've been boring audiences for decades now, and they've responded by slowly withdrawing their patronage. They're not the ones who broke the contract. They paid their money and expected entertainment; we sent them back into the night feeling bored, bullied and baffled. So what are we doing wrong?"
http://arts.guardian.co.uk/theatre/drama/story/0,,2038925,00.html
What I think we are doing wrong is not being pro-active enough. We need to take the theatre to where people already are, pubs and clubs. The same way that people would see a band in a pub, I want people to see a play. The conventions of theatre make it really easy to engage an audience (not wholly, but enough foe them to watch the damn thing), they sit there quietly and watch the show and then at the end, they get up and leave. This makes it very easy to let things slide and quite hard to see if an audience is bored. If you put most plays in a pub, then people would talk over it, ignore you, not applaud at the end, shout heckles and leave. If this is happening then you know that you are doing something wrong. This would force you to step up your game, make a more engaging show and make you learn very quickly what works in this form and what doesn't.
I don't know if any of this is true, but I do intend to find out.