Stephen Colbert to Eminem -- I Ain't Afraid of You, Punk! (VIDEO)

Stephen Colbert got in Eminem's face during a heated mano a mano showdown over retirement plans -- and things got weird ... mostly because this all went down on public access TV.

Colbert played anchorman on Monroe, Michigan's public access cable show 'Only In Monroe' and had local Michigander Marshall Mathers on as his guest.

Colbert's obviously warming up for his 'Late Show' debut -- and Em had to know this was all a joke, but his acting is hardly up to "8 Mile" standards.

But it's still worth watching ... just for Stephen and Marshall's faux face-off.