Actors, you have the hardest job since you have the most competition out there. Number one, do put your name on a screen credit at the beginning of your reel with contact info. Yes, it's on the DVD, but everything can get lost during production, so make sure your name and contact info is on everything. As to the selection of your work and the editing, you need to take a hard look at each of your pieces and decide what really show off your talent. Yes, maybe you had a role where you looked great but didn't have many chances to act, so put that fabulous image of yourself in, but don't waste time with the dialogue. Conversely, when you have a scene that shows off your acting chops, you need to feature it in the reel. In fact, open with it because the first few moments are all you have to grab the casting people. Many reels are switched off before they even get started based on that opening moment. So pick wisely. And edit them tight. Remember, nothing has to flow and make sense in context. You don't need to set up the scene, just show yourself in it and doing a fabulous job, and then cut away and get on to the next. Show range, if you do both comedy and drama. And just let the scenes end with a quick cut to black and then open with the next scene. For actors, don't use music on your reel, other than the score that might be on your sample. This is to see what you can do as an actor and your voice and presence are everything. Remember, get in and get out of scenes that show you off.
For the Director, the reel has to serve two purposes--you need to show you can create beautiful screen images and you need to show you can work with actors. You need to include both types of clips and because you are seen as someone who both initiates a project and is there all the way to conclusion, your reel has to be that polished. You need jump cuts and juxtapositions that show off your range. You need to include any scene that sets your experience apart, for instance you might have done a lot of night work or a lot of intimate physical scenes. Highlight those because you want the hiring people to know you have experience in those areas. And then you need to edit the reel perfectly, incorporating music and edits that highlight the action or scene. And do let different types of scenes be edited next to each other for contrast. You need to show the biggest range of your skills possible and it must be very well done. Remember a Director is an overseer of all creative aspects. Make your reel tight and creative.
A reel for the Cinematographer is the simplest. You need to show the beauty of everything you've shot. Even if it's raining and gusting winds, your shots need to show how you captured the beauty of the image itself. Also, you need to show that you can light and shoot people, making them look as good as possible. Yes, sometimes actors do not look good in a certain set or costume or they are bloody because it's horror, but you need to focus on how great you made them look despite the circumstances. And your reel can be the least cohesive of any other because you are showing images not content. Cut wildly between these images and pick a great song to use in the background. Edit the images to fit the music and make a gorgeous and enjoyable three minutes of film.
For Editors, you have a challenge to show off your skills. Editing is a subtle art but it packs the biggest punch. You'll need to show how you can generate a mood or a ticking clock, or get the most laughs out of any scene you cut. Let each scene begin and end with a cut. Even the editing on this reel will be judged, so make it move and be clever in your cut choices. If one scene ends with a joke, feel free to smash cut into a fight scene since it's your work a building an overall pace and tone that you want to show off. You must be in complete control of this reel and its pace.
Writers don't use reels to sell themselves, so unless you really want to make a reel, it's better to let your scripts speak for themselves.
Before you begin your reel, if you have management or representation, do consult with them and find out your perceived strengths and weaknesses and use your reel to overcome through those perceptions. And be open to making and using three or four different reels: one for comedy, drama or reality TV, for instance. The main thing is get it done and out there. Your talents are wasted without the tools you need to be hired. The film reel is one of your moist important tools.