How do they make a huge crowd in a movie?

It could be done in various ways:-

1. Different shots of small groups- Sometimes low budget movies use the same people in different shots and combine them to make a large crowd scene.

2. In developing countries like India, getting a crowd is not tough. It is possible to get a large amount of people for cheap price such as 100 Rs/day. So why worry, you can afford as much as you can. In film No One Killed Jessica, the candle light march scene is also achieve by the same method.

3. Using colorful low intensity lights, you can make to appear less people in large crowd by using colorful light to distract audience. This is very applicable in disco/club scenes.

4. THE ALL PURPOSE CREW- Sometimes crew members are also used to fill the need of crowd for the low budget films/shows.

5. Sometimes props are also used in between the crowd to make it appear large.

6. Copy paste -In movies like resident evil, doing the make up of all the zombies in a crowd is not easy and even costly. In an interview they told that they made hundreds of zombies using makeup and copied and pasted their shots in a manner that they appeared like thousands.One example of copy paste technique is - The faked crowd scene for the opening of MasterChef (source).

   

7. Fake crowd - sometimes movie uses the fake plastic crowd in distance shots, click here for details.

   

   

8. Self volunteers- its exceptional but sometimes peoples volunteers them-self without taking money. As per example of Gandhi (1982) where About 200,000 were volunteers and 94,560 were paid a small fee (under contract).(refrence)

9. And at the end CGI is also used for the requirement of large crowd.

Great question! It sort of depends on the budget, and/or the director.

Many films today use special effects to duplicate a rather small number of people into a large crowd. For example, Star Wars: Episode III, on the jungle island of Kashyyk, a war ensues of magnanimous proportion. The brainiacs at LIM accomplished this by shooting a group of about five wookie performers flailing arms and shouting war cries. Then, they scrambled the performers, had them swap weapon, and repeated the process. Then they repeated the process again, and again, and again and again. They then digitally pasted different takes at different depths, shades and speeds. This gave the illusion of a million-wookie attack force.

Film makers have been using this effect even long before the digital imaging age. They would use the same shooting method, but also physically move the performers around the expanse of a large set before superimposing film upon film.

Lower budget films will also use the few-man crowd technique, but instead of digitally copying and pasting, they will shoot the crowd in tight frames with dummy torsos above and out of the frame. This gives the illusion of a shot portraying just a small portion of a larger group.

Some directors have the budget, resources, and time to create digital crowds, but as realists, choose to hire a stampede of live extras in a massive directorial feat. Christopher Nolan is one of such realists. During the Battle of Wall Street scene in the Dark Knight rises, Nolan shot an onslaught of over 1,000 performers bashing and brawling down the snowy bottleneck. (Further look up how they recorded the Rises main theme chant!)

Another example of real-crowd display is the parade scene on Farris Bueler's Day Off. John Houghes and crew tell of sending out a mass invitation to be a part of a major feature film (impressively before Internet broadcast) and alongside a real parade, the pleasantly surprising turnout shot perfectly for this iconic scene.

In Selena, the crowd during the Houston Astrodome concert, the bottom few rows were real people and the top few rows were cardboard cut-outs disguised by flashing cameras in the upper rows.

Some crowds are also shot from real concerts or events, and the film stars are cut and pasted onto the stage, which is common in musical artist biopics. Others are completely digital.

There are many more techniques and quite creative combinations of the ones above and I'm more than certain others will develop!

Modern films use CGI techniques to create digital crowds but for older movies like the one shown in the picture filmmakers either use a naturally occurring crowd, possibly from stock footage, or simply by hiring a lot of extras through the normal channels.

To answer all your subsidiary questions start here.

I have worked as an extra for a Nike advert, and one of scenes involved a lot of people running around. They had us running from here to there, and then from somewhere else to there, etc. And they used editing to make it look like a huge crowd of us were there.

Other techniques range from hiring lots of extras (I think Lord of the Rings did this for some of their battle scenes) and, now, just using CGI and either computer generating the images or using a copy paste of the crowds with a little touch up here and there.

It is now more economical to use crowd software to fill in large crowds. Directors are sometimes hesitant to trust this process, but it generally yields a more efficient composite in the end. Shooting groups and moving them around can work, but is time consuming, weather and lighting can change, and camera motion is constrained. All these factors lead to much more fixing in post to patch together disparate shots, match light, roto edges, etc. Use your extras judiciously, as foreground elements and fill in the rest with Massive. You will be happier. I have specialized in these techniques for about a decade and they allow flexibility, multiple takes and iterative refinement of shots.