MRI's can take a long time, depending on what part of your body they are scanning. They may take anywhere from 20 minutes to 2 hours. If it's for a smaller part of your body, and therefore a quicker scan, they may not use any sedation at all. If you are extremely nervous, or claustrophobic, they may give you partial sedation. That is, they would give you medicine that makes you very relaxed, sleepy, and keeps your anxiety in check. If it's going to be a long scan, or they need to make sure you are absolutely still, they may opt for full sedation. That means that you would be put under completely, and will probably not remember any part of the scan.
An MRI is just a scan of your body. There is nothing inserted into you, unless it's a CT scan, in that case there may be an intravenous injection. Depends on who's there and what kind of results they want.
If you were in a coma, then they can perform an MRI. I've had one everyday for 3 weeks.
All you do is stand there, hold your breath (if its for lungs) and they take a picture... Thats it! :)