1. You are relieved to find out that the play is only 90 minutes long
2. You know which 'restricted view' seats are actually not very restricted
3. You know the quick route to the loos to beat the interval queue or in the case of the National, which loos to go to where there is never a queue
4. Some of the actors you get excited about seeing on stage your non-theatre friends have never heard of
5. You join discussions on Twitter about the various merits of the different theatres membership schemes.
6. You share membership schemes with theatre friends to save money.
7. Getting into town to sit on the cold step of a theatre for two hours or more to secure a day seat doesn't seem mad.
8. You'll jump on a train for two hours to attend a matinee outside London
9. You buy tickets for whole seasons in one go and get a little bit stressed
10. You discuss ticket buying trials and tribulations on Twitter.
11. Actor/director spotting in the audience becomes an almost regular occurrence and you have at least one story to tell about when you sat next to/behind/in front of someone famous
12. You recognise the theatre critics.

OK what have I missed, there must be loads more?