How to decide between short clips or a full scene in a showreel
The showreel question has been asked a lot, and I have spoken to four or five casting directors about it as well as reading interviews with many many others. The answers vary slightly but the main thing to remember is that the point of a showreel is to give someone who doesn't know you an idea of your quality and casting range. One scene won't show as much range as three scenes, therefore once you have shown them you can play an angry doctor, move on to something else. They are not watching it to enjoy the story like we do when we watch television. A showreel is an advert, a taster. Show them you at your best in various characters and leave them wanting more.
You don't want to overload them either so ideally just three or four short contrasting scenes or characters cut together simply and quickly, lasting in total no longer than five minutes and ideally closer to three. Some casting directors don't mind a short montage at the beginning but a lot hate them, so why take the risk? What does it really show them? That you can run through a forest? That you can look into the distance? That you can point a gun? That you can punch on camera? It doesn't serve a purpose, don't include one and get right to the good stuff.
If you only have one full scene, then cut out the best bit and get it online. As long as it's good quality it's better to have something than nothing. Be aware however that you need to add to it ASAP to make your showreel more effective.
Student films are a good quick way to get more footage but be selective about which schools you work for. Some of them produce very professional short films but others are completely unusable due to a mixture of bad scripts, sound problems or below par visuals.