[quote]What usually requires a prep? Just magic or can other abilities that aren't physically defend-able require them?[/quote]

Usually just magic, but anything can be prepped. Any ability you possess, unless stated as passive, and even then, can use a prep to make it stronger. That's what preps, and sometimes interchangably charges, are for. To increase the power, or other stated /thing/ of an ability. 

[quote]Is only magic that needs it?[/quote]

No, but primarily it's what its used for. Preps on other things affect different stuff. You can prep a gunshot by making it more accurate, you can prep a sword swing to make it more powerful(often doesn't matter, as some folks will just swing full force anyway). 

[quote]What sort of things can be considered not needing a prep and why?[/quote]

Mostly things that normally don't need one. You don't need to prep a gun, but you can. Passive abilities as well. Skills, which I classify as different than magic. 

[quote]What sort of descriptions can be classed as preps for magic or an ability?[/quote]

Basically anything, a sharpening focus, a complex movement, anything, so long as you note that it IS specifically being used to prepare an ability. For example:

Kei'taro's preps are noted by his tattoos beginning to glow as specific to the ability. If Alphonse wanted to sharpen the edge of his blades using Chi, then they might glow light blue. 

[quote]What happens when you do a prep, can you still defend yourself while doing it or when someone goes to attack, in order not to get hit do you have to stop? Would that just cancel out all the preps?[/quote]

You can, depending on what you need to do for a prep. If your prep requires you to focus intently for x amount of seconds, then you can't defend, or if you do, it cancels it. For me, it only cancels out the most recent of preps. So if I've already prepped twice, going for three, and you manage to stop me - I still have two preps, just not the third. This could, obviously, be different for others. 

[quote]Is a prep's time based on the power level of it and if you cast it premature, should it have a penalty? Or just lowered in power that is possible?[/quote]

The longer you prep, the strong your attack will be/the sharper your accuracy(if using a gun), etc., etc. If you use it with one prep, it won't be as strong as someone who casts it with two preps. Now, if you have it written that you can cast your magic without a prep, but it has a cost, be sure to factor that in as well.

[quote]How do you determine when an interruption is successful?[/quote]

An interrupt is successful when it's logical, basically. If someone is trying to do too many actions in a post, you can interrupt between any one of them if possible. If I'm throwing 100 punches(Stated in a post), you can stop me at 2, 5, 43, 78, 99, whichever number you want if possible. If you can successfully do that, without metagaming, godmodding, or otherwise cheating, then you're successful. It basically will cancel out the entirety of my post after the point in which you interrupt me. So if you stop me at 43, then I have 57 punches that just aren't there anymore. 

All answers are based on my own interpretation of T1, [@Skallagrim], [@Melonhead], DJ, even [@Vordak] may have a different view on each question. [@Fallenreaper]