Here's a behind the scenes photo from the set of Star Wars: Episode VI - The Return of the Jedi featuring Boba Fett without his helmet. The photo has completely ruined my vision of what Fett actually looks like under the helmet. He is the least badass person they could have gotten to put on that costume. I'm not really sure, but there's a chance that that could be Frank Oz. I included another behind the scenes shot from the same movie below. I don't think it is, and I couldn't find any information that Oz ever stood in as Boba Fett. All I know is that now, every time I see Boba Fett I'm going to think that of that face behind the mask. Thanks to Kristian Odland for the tip!
Updated Information: This is Don Bies, who is one of two ILM employee's that played Fett while George Lucas was shooting the special edition films. The guy was obviously still clearing living in the 70s with that mustache and glasses. Thanks to GT reader Brian Moman for the insight!