Roger Nores appears in our new documentary, "TMZ Investigates: Liam Payne: Who's to Blame?" -- which airs Monday night at 8 PM ET on FOX -- says he was with Liam on and off the day he fell from the hotel balcony to his death.

Nores says, he checked on Liam three times during the day, and when he left an hour before the fatal fall Liam seemed happy and fine, although a little tipsy.

But authorities say, Liam was unhinged as the day progressed ... not only drinking heavily and doing cocaine, but texting Roger, "Can you get 6 grams?"

And then, there is Bret Watson, a Chicago man who was at the hotel for his wedding, who says his wedding party saw Payne all day long... Adding the singer looked more and more intoxicated as the day progressed.

Watson says, a little more than a half hour before Liam fell from the balcony, the singer was lying on a sofa looking out of it when he saw something on his computer that infuriated him. Watson says Liam jumped up in a rage and slammed the laptop to the ground.

As we reported, Liam created enough of a disturbance that hotel employees grabbed him by the arms and legs and took him back to his room, where they left him alone.

The documentary includes an interview with a waiter who is also in the crosshairs of authorities for supplying Liam with cocaine.