Eminem Shocks Fans With Surprise Appearance at Documentary Premiere in NYC

The real Slim Shady stood up in NYC ... making a surprise appearance at his documentary, which had fans losing themselves.

It all went down Wednesday night on The Rooftop at Pier 17, when Eminem showed up to a sold-out theater full of his stans -- fitting, since that's also the title of his newly produced documentary.

In the video .... Eminem talks about the impact his song "Stan" had on people and how he didn't realize it would be such a big deal when he was writing it.

Eminem had to pause a few times to respond to fans who kept shouting over him ... they said they loved him and he saved their lives ... further underscoring how much "Stan" means to some folks.

The film takes a deep dive into Eminem's career through the eyes of his biggest fans -- featuring personal interviews and never-before-seen footage.

Of course, the rapper's iconic 2000 hit, "Stan" -- about a dangerously obsessed fan -- serves as the throughline of the documentary, which spotlights the passionate supporters who've grown with him over the years.

Earlier in the day, there was a look-alike contest in SoHo, where fans came dressed up like Eminem's character in the "Stan" music video ... showing off bleached blonde hair and white tanks.