It’s a good question to ask since those in charge usually need to have a permit to film in crowded areas or pretty much anywhere. But there are a couple of ways to do it since there are times when it’s easier and more cost-effective just to build a set and stage the scene within the controlled set instead of having to go to the massive amount of effort and trouble that comes with the other method. Gaining permits, blocking off streets, getting the police to cordon off certain areas, making sure that no one decides to just push past barriers and do their own thing, all of that takes money, material, and time, and of course, time is just more money since all of these people that are putting into this project need to be paid and taken care of. And then there are the makeup trailers, the catering, the equipment trucks, and so on, and so on, and so forth. There’s a lot that goes into a movie and when filming in a crowded area one has to think that everything has to be just right or things are going to fall apart in a big hurry. There is a possibility of filming without gaining the needed permits, but this tends to be ruined the first time some overzealous fan or random individual happens to notice a camera or that there are one or more stars in the shot trying to do something. There have been a few happy accidents in the movies while shooting a film now and then, but otherwise, the process is fairly laborious and requires a lot of expert timing and planning.
Creating a set piece that’s meant to represent a certain area that might be kind of tough to film in is a good idea but it does cost a good amount of money and there are setbacks to it since the set can’t move and it’s not able to show anything beyond it. These are just a few difficulties that filmmakers have to deal with since if they can’t get a shot in the crowded area as they want, then they have to have to find a way to improvise that will look professional enough but won’t cause an arm and a leg. Big studios can afford this more often than not but those that are still working their way up the ladder and those that are basically making movies for the sheer pleasure of it have to deal with a great amount of difficulty that comes from not being able to obtain the necessary permits, not being able to build a set that’s bound to feel realistic, and not being able to keep people from saying ‘hey look, a camera!’. Bigger studios with massive budgets can do this quite easily, but even the big studios sometimes see the sense in creating a set rather than trying to get the real world to cooperate since realistically there are dozens of variables that can occur when making a movie, and the weather is just one of them. People are usually bound and determined to be a part of something if they happen to see it going on, which presents another challenge that has to be dealt with.
Extras are, of course, a necessity for many movies since unless a movie is going to be filmed with a minimum of actors there will need to be people walking about, people driving, and people basically just going about their everyday lives. After all, a lot of movies need the realism in order to appear believable, while it’s a cool effect at times to see the streets of New York or LA, or any other big, crowded city absolutely empty and devoid of life, such as it happened in The Devils Advocate at one point, it’s not entirely realistic and it would take a massive effort to make happen since places like this simply have too many people in them to make it possible for more than a few shots at best. But when you think of all the effort that goes into those few shots, you get a better understanding of what it really takes to make it happen and make it look absolutely perfect. Shooting in crowded areas is a risk no matter how carefully planned it is since just about anything could ruin the shot unless a director or editor can make it work. But when it comes down to fabricating the set and using it to get what’s needed, or obtaining the permits and going through the process of closing off streets, hiring extras, and paying for a small army of people to make sure the set is clear and ready, sometimes it sounds like a lot of work for very little gain. But hey, that’s show business.