If you're going to go big and make a designated room for your home theatre, you will need to make sure that it does not impact on the rest of the house as to minimise disturbances. If you have the chance to do so, pick a room that is spacious and separated from busier areas of the home where others could get disturbed from the noise, or implement insulation to absorb some of the noise impact.
For those who are conservative with their space, you may be able to combine this experience by replicating such an experience in your lounge. With the correct sound system, some comfy seats and being conscious that the room is not a movie theatre in a lounge to ensure it can still be used as a standard living room without any issues.
Depending on how you wish to lay out your home theatre, you may opt for a projector over a TV to give that traditional cinema experience feel. If using a projector, you may want to pay attention to the lumens, as the higher, the brighter the output, meaning that brighter ones could be used in the day on a suitable background or wall, whilst lower ones can only really be seen properly in the dark.
To further compact your space and keep things tidy, you may use a roll down screen give your room a clean feel to it, but some way wish to use a fixed screen so that there is no variance in the material from being put up and down over time, or even the tension on such.
Depending on how you enjoy your videos, you may find yourself straying away from using typical systems such as a DVD or Blu-Ray player and instead using a system like the Apple TV, Fire Stick/TV or Google's Chromecast (4K option available for those lucky enough to get a 4K projector!). If you still prefer to keep your own film and TV collections to hand on your computer, you may be able to run them from a designated media centre program or device using a cable connection to the projector, whilst others above connect wireless and can allow you to stream the contents of such videos direct from your computer.
Can everyone set the screen comfortably from where they are sitting, and are they comfy? Making sure your seating arrangement can accomodate everyone can sometimes be a challenge, if you have made a home theatre, you may choose to create a platform to raise up a second row of chairs to give that authentic cinema feel. Though, you may choose to crossover home theatre and living room by creating a raised platform for a second sofa to go on top of. Bonus points if you are able to find a chair with a built in storage space for snacks, drinks, or to keep a box of tissues to hand for those heart string-tugging moments in your favourite films (The editor still can't watch Lion King without crying).