When your landlord is also your roommate, the relationship is a bit more murky than a clear tenant/landlord relationship. The Move Channel.com has some helpful advice for successfully renting a room from a friend.
Make sure you are aware of what the rules are for how your roommate/landlord wants the apartment treated. Make sure you know exactly what you have access to, how food is shared or not shared, etc.
Ask how the bills will be paid and what the split for them will be. Will your landlord/roomie pay the bills and then you'll pay them back? Will there just be an allowance for utilities built into your rent? Get clear on how things will work before you agree to move in.
Speaking of rent, don't fall into the trap of thinking it's your friend so you don't have to pay your rent on time. They still have a mortgage and bills to pay, so turn in your rent payment on the day it's due.
If you're living with a romantically involved couple, make sure you have a plan for if they break up. Also, don't get involved in any arguments they have and never take sides or commisserate with either party!
Lastly, if your friend is the person you are renting from, they may not give you a formal lease to sign. At the very least, write down any questions you have and have both of you sign it so you can refer to it if there are any disagreements over particulars down the road.