Williams was certainly a violent guy in his past. David Simon claims that he tortured 3 people in an apartment building, though Williams says that this was overblown.

Williams blames his violence on his upbringing, even revealing that

“psychiatrists have diagnosed his violent tendencies to be related to persistent pain from a variety of near-fatal encounters”

We actually don’t see a lot of violence from Avon because he’s at the top of the organization when we meet him and can hire hitmen, but it’s assumed he did some serious shit in order to gain the respect of hardened soldiers like Charles and Wee-Bey.

Marlo’s personality was mostly based on Timmirror, but his name also comes from another Baltimore dealer, Marlow Bates.

The South Baltimore Westport area and West Baltimore Murphy Homes housing projects, to be specific. [source]

https://www.youtube.com/watch?v=FNkT1tZR_iA

https://www.youtube.com/watch?v=UTDQ-nzNIgY

https://www.youtube.com/watch?v=YwogM2DL99o

The novel was The Wolf Trap, an urban fiction novel that actually received very favorable reviews

http://www.amazon.com/The-Wolf-Trap-Dennis-Wise/dp/0975393820

Cutty actually grows close to the character inspired by Collins, Wee-Bey, while the two were jailing together.

Jackson also claims that the police would follow him to his classes, which we see when McNulty tracks String to his economics class.

On the show, Stringer owns an office supplies store, a strip club called Orlando’s, and attempts to get into the real estate business.