How to put together an effective showreel
The key to putting together an effective showreel is all about context. Once you have gathered enough material, it is time to assemble your showreel. There are many companies dedicated to producing showreels for actors but I cannot speak for any, as I find them cost prohibitive and prefer the greater control I get from making my own reel.
When editing a showreel, it is important to remember the context in which it exists. The showreel is there to supplement your headshot and cv, and to give potential employers a look at you in action. As such, if your showreel is being viewed, it is likely that you are already under consideration and the casting directors are viewing it, wanting to be impressed.
When editing a showreel what you are doing is essentially creating an advert where you are the product being sold. Therefore it is important to remember your target audience. So ideally this will be casting directors who are already thinking of you for a part. The casting process involves casting nets as far and wide as possible and therefore is very time consuming. Many casting directors cite around two minutes as the cut-off point for showreels, with some even less than that. Consequently, structured editing somewhat goes out the window and it is best to create a top heavy showreel. Start with your best piece and then progress from there. Many actors start their showreels with a musical montage but casting directors tend to prefer a reel that gets straight to the point.
In terms of best piece, we return once again to context. This is your showreel so the best piece is the piece in which you are at your best. It doesn't matter about the calibre of actors around you, or the standard of the camera work. As long as you are clearly visible, audible and delivering a good performance that is all that matters.
Afterwards it is time to choose a second clip, ideally this will be something to contrast the first, showing the range of the actor. Comedic and drama, classic and contemporary, I'm sure you know the drill and already have such balance in your monologue selection.
Knowing that the attention span of a viewer generally sits at around the two minute mark, often the majority of the showreel is taken up with two clips. If this is the case then make sure these two clips show you to the best of your ability. Of course, your showreel doesn't have to be two minutes, anything up to about five is fine, but be aware that it is unlikely more will be watched and don't squander your best work at the end of a lengthy reel.