When a comedy is filmed in front of a studio audience, the audience laughs at each joke and that laughter is included in the finished episode.
But presumably there would be many scenes with multiple takes, and so the same jokes would be repeated. Given that jokes get less and less funny with every telling, how come the audience "keeps up" the laughter for the particular take that gets used in the episode?
There will be separate microphones capturing the audience laughter. This means that during editing, laughter from one take can be used for a different take.
Usually the audience laughter is recorded such that it can be edited into the soundtrack, known as the laugh track. Many shows have gone so far as to use so-called "canned laughter" - which was pre-recorded and sold to studios.
In addition to the editing techniques mentioned, many shows with a live audience also have a comedian who serves to warm-up the audience. This is common on both talk shows and on sitcom tapings. It makes the audience feel looser and eases them into engaging later on when they are watching the material that is being taped.
Based on my own experiences as a member on several live audiences and from discussions with a friend who is a writer and producer ...
The audience is explicitly told that they need to react to re-takes as if you hadn't already heard the joke.
Don't flub the lines. Most sitcom actors are also stage actors, used to live theatre. Direction, timing, and blocking have all been worked out in rehearsals, and, with three or four simultaneous cameras, you've got some flexibility to cut around minor technical mistakes. There may not be as many retakes as you might expect.
A warm-up comedian keeps the audience energy level up between scenes or other delays.
Don't go long. A half-hour comedy, when all goes well, keeps the audience only 60-90 minutes.
Feed the audience sugar. Often, before the last scene of the show, the ushers will give out candy bars so that the audience members have a burst of blood sugar right as they're capturing that final joke.
Laugh tracks are "sweetened." They can blend (mix) the live audience reaction with their reactions from other takes (or even other parts of the show). They can also mix in guffaws and laughter from libraries of audience recordings.
Change the joke. On some shows, the writers are on set during the taping, will gauge the audience reaction and provide alternate punchlines, so the re-take is familiar but has a fresh punchline.
Cut the joke. I once watched an actor get tongue tied again and again on the punchline for a throwaway joke from a sitcom opening. After eight or so retakes, they just switched to shooting another joke (which I think was written for a future episode). Later, I saw the original joke attached in a different episode.
Hide the reveal. This isn't so much about retakes but about jokes based in the sudden visual revelation of a situation. For example, you see the character in bed talking to whom you assume is their spouse, then the camera pulls back and you see they're actually talking to their pet. I've seen this done by blocking the live audience's view of the set with a curtain and having them watch the restricted camera view on monitors. At the moment the camera pulls back to reveal the Saint Bernard on the bed, the curtain is dropped so that the laugh it timed just right.
Pre-shoot technically difficult scenes. Sitcoms are generally shot in sequence for the benefit of the audience. Old shows used to keep almost all of the action on one or two sets that fit in front of an audience. Modern shows might have a dozen sets, outdoor shots, montages, and even complex visual effects shots that make it impractical to shoot everything in front of the audience. So the hard bits are pre-recorded and edited. The audience is shown the pre-recorded sequences on monitors and their laughter is recorded. No retakes there.
Improvise.
Long ago, I saw a shooting of Cheers. They were doing an episode and a half in one night, in order to make up for a holiday. The first episode started more than an hour late. And that was with the tongue twister opening joke that they punted after many takes. Lots of little things went wrong, so there were extra delays. By the final scene of the full episode, the audience energy was fading. And, for reasons I don't recall, they kept doing takes. Hardly anyone laughed. The warm-up comic tried everything. The last joke involved Norm (George Wendt) and Cliff (John Ratzenberger), we'd already heard it many times, some in the audience were literally asleep. So, in that last take, just at the punchline we all already knew by heart, John Ratzenberger started riffing--in character--telling raunchy jokes, making up hilarious stuff. This got the audience's attention. The audience, the crew, and the cast were all laughing hard. Then Ratzenberger pointed at the director, said "keep rolling", and he delivered the scripted line.
By the time they finished the first episode, it was approaching 11 PM. They wanted us to stay for the first half of the next episode. But many of the audience members trickled out. The sound people had the ushers consolidate the audience into one quarter of the seating area in order to concentrate our laughter in one audience microphone and they shut off the rest. They sent up more candy bars.
They started the second episode and everything went fine. Then there was a break for a costume change, and everything ground to a halt. For a very, very long time. At last, they shot another scene, that--if I recall--didn't even involve the character with the unusual costume. In the scene, there was an after-hours party in the bar, so there were these catering trays with sandwiches all over the set. As soon as the scene was over, the director sent the sandwiches up to the dozen or so of us still in the audience, clearly a bribe to keep us awake and laughing. At last, we saw the midpoint scene. I don't think we got off the lot until after 1 AM.
When you go in as part of a live audience the producer and director and set director talk to you and make you feel like you're going to be part of the show so when they tell the joke they tell the crowd to applaud and even though they've heard the joke several times they're told that every time you're asked to applaud you do it like it's the first time if for some reason it doesn't work then they do use what's called canned laughter.
I went to a sitcom taping once. If I recall correctly (it was about 20 years ago, "Just Shoot Me"), there were rarely more than 2 or 3 takes of each scene.
In addition, they usually tried different jokes in each take. The main point of doing all these takes was to see which jokes worked best, it was rarely because something went wrong and they needed to redo it. So the audience didn't always know what's coming, and didn't get bored by numerous repeats. We laughed at them all.
And as others have mentioned, there's a warm-up comedian keeping the audience energized during all the downtime (the taping took almost 3 hours).
One factor is that comic actors will usually stop as soon as it's clear that a retake is needed, to avoid spoiling any more of the scene than necessary.
(This is of course the opposite of non-live recording, where actors generally keep going until they hear “Cut”, to maximise the usable footage.)
Whilst there are re-records, in a lot of cases there will be a warm-up guy keeping the audience in a good mood so that they laugh a lot for the jokes, and sometimes the audience is just quite happy just to be there. When I went to one (Red Dwarf), most scenes had several takes yet the warm-up guy kept us all in good spirits.
Also, the audience laughter can be edited or taken from different takes for the desired effect. The version of the take you see may be from a different take than the laughter is from, as most of the time it will be possible for the editing department to split them and "mix and match", as it were.
EDIT - additionally, some bits may not be "recorded" in front of a live studio audience, for various reasons (e.g. location shoot, or having a joke which wouldn't work with the studio audience present), but instead be recorded beforehand and then shown to a live studio audience, thus negating the need for several takes in front of the audience.
A few years ago, I was invited to be in the "audience" for a multi-camera show called "Partners," starring Kelsey Grammer and Martin Lawrence. What was unusual is that the show had already been filmed and edited, but it hadn't aired yet. The reactions of the live audience, watching the recorded show in a screening room, were to be added before airing. I didn't go to the screening. The show only lasted 10 episodes, so I imagine the audience reaction at the screenings was not helpful!
This isn't common. The only other sitcom I can think of that did this was "How I Met Your Mother," which was far more successful (208 episodes).
I am not an expert, but I have sometimes seen episodes of a rather unusual sitcom called Just Roll With It (2019-2021).
They definately have a studio audience, since closes ups of audience members reactng are seen, and even sometimes views of the stands full of audience members. The cast members mingle with the audience at the end of the show.
And from what I have seen the audience seems to enjoy the experience. Maybe some or all of the audience are extras paid to pretend to enjoy the show, or maybe somebody keeps the audience in a happy mood between takes as other answers suggest.
And possibly there aren't many retakes of botched scenes, to reshoot and bore the audience, since the gimmick of the show is that it will periodically be stopped and changes, voted on by the audience, made to the plot, usually involving gross experiences for the actors. So presumably the director doesn't complain if, for example, glop spills on an actor in a non photogenic way, and demand a reshoot.
Though no doubt having many retakes of a particularly unpleasant experience would be a good prank to pull on an actor, if it wouldn't be too expensive.
Other shows don't have the same gimmick as Just Roll With It. But if - and I don't know if that is the case - the audiences of Just Roll With It are typical of televiscion studio audiences in general, it would appear that most of them don't get too bored during filming.
A lot of Sitcoms haven't use live studio audiences in years. I Love Lucy was the first television series to be filmed in front of an live audience. This was made possible by the idea of Desi Arnaz to use multiple cameras. While sitcoms like Seinfeld and FRIENDS and such still use live audience, they've dipped in utilization, namely because the proliferation of Camera phones allows people to essentially leak potential stories before it has aired. Live audiences are usually not the main source of laughter/awws/sounds in show, They may be audio recorded, then played back at points plot.
