I'm working on a beginner and intermediate demo/smoothing tutorial specifically for TF2, so hopefully I'll be able to answer any questions you have with that.
Anyways, whenever you load up a demo (in console: playdemo "your_demo_name_here_without_quotes") type Shift+F2 to load up the handy demoui tool. Think of it as your remote control for viewing the demo.
In order to move freely in the demo, you need to press the "drive" key. However, unless you where in spectate, your entire player model will move with you, as you will are unable to see yourself in a demo aside from the firstperson and thirdperson viewpoint.
As Cameron
said, demos can be pretty complex, and they are very very unstable. Feel free to PM me with any other questions though. Also let me know if none of this makes any sense and I'll try to clear it up for you.