Marshall, testifying in the first-degree murder trial Friday of Willie Clark, 26, recalled how he and Broncos defensive end Elvis Dumervil approached Clark and another man to try and calm a developing conflict inside the party after Marshall had earlier talked a bouncer into letting the men into the event.
Clark also faces 16 counts of attempted murder and other charges. Two others were wounded in the shooting.
Prosecutors say he fired the fatal shots into Williams' rented limo. Clark maintains he is innocent; his defense attorneys argue he's a scapegoat. Marshall identified Clark in court as being among those involved in the confrontation.
"I got in the way of the skinny guy, and Elvis got in the way of the guy with the dark sunglasses," Marshall said of men reportedly identified as Daniel "Ponytail" Harris and Clark, according to The Denver Post. "I told them, 'We got all these bottles of champagne up here, all these women, it's New Year's -- just party with us.' After everything died down a bit, that was pretty much the end of it. They stepped back, and the party went on."
Marshall said the men had been throwing up gang signs and harassing a table of people that included Williams, according to The Post.
But later, Marshall said, outside the Shelter nightclub in downtown Denver, he saw the conflict had resumed, with Clark and Harris again making gang signs at Willams' group, which was now in a rented stretch-limousine Hummer.
"This is when I got angry a little bit and my first words were, 'God [expletive] man, I offered you guys drinks tonight to party with us.' There was a lot of cursing and a lot of words being exchanged," Marshall said, according to the newspaper report.
Clark pulled up his shirt slightly as if to show Marshall he was carrying a gun, the receiver said, according to The Post.
"I said, 'Man, you got no [expletive] gun,' " Marshall said. "I don't know if it was a gun or not. I assumed he didn't have a gun because he was holding up his shirt and I didn't see one.
"I kind of got angry at the time and probably escalated the situation, but I got angry because it was New Year's and we were trying to have fun."
Marshall said he was "sure" he made gestures in return before Harris hit him in the jaw after Marshall had slipped on ice trying to climb over a snowbank.
Marshall said Clark and Harris then fled.
"The first thing I thought of is maybe they are running to go get a gun," Marshall said, according to The Post.
But he said he didn't see who got into the SUV used in the shooting.
"No, I just seen them scatter," he said, referring to the melee's aftermath.
Marshall was at times emotional when he testified, recalling how he later saw former teammate Javon Walker in a bloodstained shirt at the hospital, clutching a necklace that belonged to Williams.
"JWalk just kept looking at this chain, he wouldn't let no one hold the chain," he said. "It was scary; there was blood on that chain."
Prosecutors also showed surveillance video of outside the club while Marshall was testifying. They said several people were flashing gang signs.
Marshall, who was not in the limo when Williams was shot, said he later got a phone call from someone telling him what had happened.
Information from The Associated Press was used in this report.