To start with, then: other audience members. I personally think that the people sat around you make a huge difference to how much you enjoy a performance. If you're watching Les Misérables (and if not, why not?) but the atmosphere of the audience is disconnected, you are far less likely to become lost in the (stunning) show. On the other hand, if you are watching Kinky Boots (oh please do!) and the audience is cheering and applauding at any given opportunity, you might not be able to avoid the infectiously good mood.
Of course, it is difficult to gauge when an audience is going to be good or not. I think that, often, evening performances are received more emphatically than matinees – but why? Perhaps it is because the house is more full; perhaps people just take a day to become worked up for a show – I really don't know. And this is only really a rule of thumb anyway: when I went to see, for example, Phantom of the Opera (another exceptionally brilliant production), it was a matinee performance, and yet the atmosphere was fantastic. But when I compare seeing Wicked in the evening and in the day (is it possible to see this show too many times?), I think that the evening performance did get a better response. This bears no reflection whatsoever on the shows themselves, but rather the audiences.
There are times when a couple of audience members can change the mood of the show by themselves. If somebody is a little too loud, they can put the audience in an irritable mood. The extent to which this effects your overall enjoyment of the show depends – but it always has some effect. Sometimes though, the same one or two people can make the mood a lot better: perhaps it was that one person who started the standing ovation, or the applause, or maybe the person who audibly caught their breath when that character died …
So to what extent does our overall enjoyment of a show depend on the audience amongst whom we are sat? I would suggest that this depends on the show itself. Perhaps some shows rely more on the energy of the audience than others. Sometimes the audience itself might not even make a noticeable impact. Even when there is a marked difference, a good show would be hard to ruin just because of a disengaged audience; equally, a bad show would be difficult to enjoy just because of a good audience. I suppose it is the difference between enjoying the show from a distance, and becoming completely absorbed within the production. Perhaps that, then, is the power of the audience.
When it comes to the people on stage, audiences have a far more distinctive effect. Particularly so with comedy; there has to be a level of energy between the actors and the audience. If the audience doesn't participate in this at all, performing can begin to feel a little forced (at least, I have found this in amateur productions). This works both ways: a particularly good audience is sure to increase the energy levels of the actors.
The impact an audience has on actors is perhaps not the most important point here: it is interesting to consider, as an unqualified idea of its own, that the audience does not play a passive role.
But when did we start assuming that audiences played a passive role at all? Looking back at, for example, Shakespearean plays, it is evident that audience involvement used to be a lot more explicit. Perhaps, then, a better way of looking at audience involvement is this: audiences are (usually) less involved within a theatre than they used to be, but this involvement has never been completely lost.
In short, do not just 'sit back and relax'. Engage with a show, and it will be improved on two accounts: the general ambience, and the essential link between actor and audience. In the wise words of Sarah Wayne Callies:
“An actor without an audience is rehearsing.”