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Attending a Sitcom Taping is a lot of fun, even if you don't know the show. It's like watching live theatre, but you get to see the actors flub their lines and try again, or experiment with different variations on a theme.
Most sitcoms are taped weekdays with call times between 3 and 7 pm. There used to be a taping season, but now you can almost always find something taping.
After standing in line for an hour or so, you go through security, and are checked for recording devices (including cell phones, leave them in the car), among other things, and are shown into a studio. Seats are filled in order. There are usually only about 10 rows of seats, so no one is far from the action.
Once you're seated, the warm-up person, usually a comedian, comes out to get the audience in a laughing mood (which you might not be, after standing in the sun for hours). They also keep you entertained during pauses when the cast is changing costumes or the crew is changing camera angles from one part of the set to another.
The entertainment is often interactive, so if you do impressions, be prepared to volunteer. If you're lucky, cast members might come up to the audience to say hi between scenes.
Parts of the story that take place in other locations may have been pre-recorded, so they'll show you the missing pieces on a TV screen.
Although they aim to get the show taped within a certain amount of time - say two hours - unforeseen glitches can occasionally drag the production out for much longer. You are expected to stay for the duration.
Sitcom audiences do not usually appear on screen, so the dress code is more lax. Shorts and t-shirts are usually ok, but it's freezing in the studios, so I would personally wear long pants and bring a jacket or sweater.
The age limit for most adult sitcoms is 18.
See Audience Etiquette for more tips.