The Wire co-star Clarke Peters told BBC Radio 4's Today programme: "He was a generous actor. It wasn't me, me, me. It was us, us, us."
Film-maker Spike Lee described Williams on Instagram as "his brother" and said he "was shook" at the news.
The Wire creator David Simon said he was "a fine man and a rare talent".
Alongside a picture of Williams, Simon wrote that he was "too gutted right now to say all that ought to be said", adding: "On our journey together he always deserved the best words. And today those words won't come."
The star was found dead in his New York apartment on Monday. US media quoted law enforcement sources as saying he died from a suspected drug overdose, but that has not been confirmed.
Williams, who was nominated for three Emmys, had openly discussed his struggles with drugs over the years.
In The Wire, he played Omar Little, a gay, streetwise robber of drug gangs.