At its very simplest the power of a theatre critic can be incredible.
Theatre is the most collaborative of all the arts. Productions will often have at least two or three and often ten or twenty or thirty or forty people working on them for months if not years.
And then, generally after only a few full rehearsals and, if the company are lucky, a preview or two, the critics are invited in and asked to judge and then share that judgement with people who are thinking about buying tickets.
There are stories of powerful critics like Ben Brantley from the New York Times whose reviews can shut a show down or sell it out.
I've worked on shows that have received five star reviews and even one that made Ben Brantley cry, and I've worked on shows that have been laughing stocks and mercilessly pilloried by the media. The former is a lot more fun than the latter.
I've also worked on shows that have been performed at the Edinburgh Festival Fringe, the world's largest arts festival. In 2012 there were 2,695 shows at the Fringe and around 1,000 journalists. The Fringe is as much a festival of arts journalism and criticism as it is of the arts.
At the Fringe the reputation and reviews of one paper stands above all others. Every year The Scotsman commits to reviewing every new production at the Fringe. Like Ben Brantley they have a legend attached to them: five stars can sell a show out. In an incredibly busy market place the competition to get five stars can be intense.
Joyce McMillan is the Scotsman's theatre critic and has been reviewing theatre in Scotland for the last 30 years.
Joyce's review of TEAM's Dance Marathon at the 2010 Fringe was a prime example of her writing. It combines a brief synopsis of the show, a personal reflection and a study of what that shows says about society and the human experience. And it does all this in around 300 words.
"... it's a measure of the force of this show that like our society itself, in the end it enforces a savage division of experience. Some leave triumphant, many are exhilarated, most are OK; and some are just losers, shattered, excluded, and lost."
"The way the audience responds to a play and the way the whole event happens between the play and the audience is a societal thing and that's really what interests me." Joyce says when I ask her how she reviews a show.
"I'm not interested in theatre for its own sake. I'm interested in it as a an imaginative, free flowing public forum in which people can share experience with other people in the moment, in the time we live in now. And that's what I try to write about. I like to connect it to the things that it's saying and to the society that has shaped the audience."
I've known people who've been reviewed by Joyce. I've seen very clever, very professional people furious with rage at Joyce's opinion. I ask her if she's ever suffered because of a bad review.
"Someone once just chucked a half pint of lager over me in the Traverse bar about 25 years ago. Can't remember why, can't remember even what review it was. Once or twice I was banned from professional theatres; once from the Byre and once from Perth for giving them a string of bad reviews. That was quite an extreme reaction I thought. Particularly as the show's in question were quite bad. They must have known themselves. Probably that's why."
Although Joyce tells me later that a reviewer's opinion has to be honest she does confess to going easy on the people on stage:
"I think if you look right through all my reviews over the years you'll probably find less than two figures worth of times when I've actually put the boot into an actor for a bad performance by name, because I just feel the vulnerability of performers at the moment when they open a production and then have to get reviewed immediately is very, very great and I actually try to avoid spiteful or witty negative reviewing of actors because I just think it's too much."
The directors and the writers of a bad play don't get the same dispensation. And if that means Joyce has to risk a half pint in the face then so be it.
"Quite a lot of the plays that are done aren't that good and you really just have to say so."
What about the need for professional critics? With so many of us sharing opinions on Twitter and Facebook and establishing our own blogs what makes the professional critic different from anyone with a smart phone and a theatre ticket?
"There's no sense that [the professional critic's] views have any more value than anyone else's. But at the same time if you express them well and with a lot of background knowledge and can just take on a lot of context for a production then there's value in that and people enjoy reading it and take it seriously, and just like in any activity if you do it for long enough and people appreciate it the way you do it people begin to respect you for it."
"There is something about being at the theatre every night that fires you straight into the heartland of human experience in a very safe way. You can precariously experience very extreme situations every night of the week without putting yourself at any actual risk at all. It takes a certain kind of personality to do that. You need a lot of energy and you need to really care about it but at the same time you need a certain emotional distance or reserve to be able to spend most of your evenings doing that."
Like the plays Joyce watches she is aware of the place in society and in history that the theatre critic finds themselves.
"I think it's quite a profound activity being a theatre critic. There's a little thread of theatre critics in history tangling themselves in politics. I think it's a pretty ancient profession. It's not quite prostitution but it has same degree of antiquity. "
Follow Andrew Learmonth on Twitter: www.twitter.com/@andrewlearmont