Well, that's not exactly right anymore.
These days, the danger worry of a fire or a film break is extremely low. Every film distributed now is made of a durable plastic alloy and is called "safety film". It does not burn - it can only melt, which doesn't happen very often either. The film is also very sturdy and, rather than snapping if it gets caught or tangled in the projector, it is more likely to damage the projector itself or some of the surrounding equipment.
Most theaters in the 21st century do not use the commonly pictured reel-to-reel projectors for 35mm films. Films are still broken down into a number of reels (generally 5 or 6 for a 90-minute film, and more if the movie is longer) both to accomodate theaters still using old projection methods and to simply make the movies easier to transport from location to location. Once the film arrives at the theater - usually in a couple of large orange or silver "cans" - the projectionist must bring the movie into the booth and, if the theater is equipped with the now-common "platter" method of showing movies, he must "build up" the film. The platter method uses several huge round surfaces stacked vertically (with spaces in between), all connected to the "tree". The tree is equipped with all sorts of rollers around and through which film can weave its way. When the projectionist builds up the film onto a platter, he or she takes a reel from the can and places it on the Make-Up Table (MUT) and proceeds to wind it onto the platter. When it reaches the end of the reel, he or she must cut off the "foot" of the reel (when on individual reels, several feet of extra film goes on the beginning - the "head" - and end - the "foot" - of each reel which contains no movie images, just information about the reel, the movie, or simply blackness), grab the next reel from the can, cut off the head of that one, and splice the first image of the next reel to the last image of the previous one. This goes on until all of the reels are built up into one immense circle of film lying flat on the platter. When projecting this sort of movie, the film comes off of the platter starting in the center, goes around and through several of the rollers, and enters the film projector from the top. It is "threaded" through the projector, emerges out of the bottom, goes through some more rollers, and returns to another platter protruding from the tree. The projectionist then must simply press the "START" button on a nearby mechanism and the whole apparatus will start going, winding from the dispensing platter onto the receiving platter all the way through the movie until it is over.
This automated system means that the projectionist will usually simply press the button, sometimes check the focus and framing of the movie on the screen (sometimes not), and then leave the booth to hang out in the lobby with his mates until the movie ends - or, in the case of the ridiculous multiplexes - rush over to another booth to start another movie five minutes later. So, if you're watching a movie and it is out of focus - and it remains so after the first several minutes - go find a theater employee and tell them because odds are the projectionist has no idea and won't have an idea any time soon unless he's told - after which he will feel slightly incompetent and check the focus for the rest of the day before returning to his sloppy methods tomorrow.