Since 2003, Sheen had starred in "Two and a Half Men" as Charlie Harper, a perpetual bad boy who mimicked Sheen's real-life persona. Last year, Sheen reportedly inked a deal to make $2 million per episode.
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"Two and a Half Men" went on hiatus in January following Sheen's hospitalization and return to rehab. Sheen has publicly slammed Lorre, Warner Bros. and CBS in a series of recent interviews, claiming he's ready to return to work and would sue for being barred from the show.
According to Spiegel's letter on behalf of Warner Bros., Singer sent the studio "eight pages of diatribe and threats of litigation" on behalf of Sheen.
"Your letters paint a picture of Mr. Sheen's condition and conduct that has no connection to reality," Spiegel wrote. "Let us set the record straight."
Sheen's "inability to perform the essential duties of his position," Spiegel added, "included Mr. Sheen's physical appearance, inability to deliver lines, inability to collaborate creatively with staff and crew, inability to work with the executive producers, inflammatory comments poisoning key working relationships, and frustration of the show's creative environment by the public spectacle of his self-inflicted disintigration."
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Sources connected to Sheen told TMZ.com that the actor's contract has a "Michael J. Fox" clause, which allow him get his salary even if he's no longer on the show.
TMZ's sources claim the clause is same as when Fox was starring in the show "Spin City" and his contract said he would get paid as long as the show continued to be produced, whether he was still on or off the sitcom.
Since CBS and Warner Bros. canceled the remainder of "Two and a Half Men's" season, Sheen has been everywhere, coining catchphrases in TV sit-downs ("winning," "tiger blood," "Adonis DNA"), tweeting about his philosophies and live-streaming his life online.
Over the weekend, Sheen launched a Web show on Ustream called "Sheen's Korner," a 50-minute homespun takeoff on a baseball post-game show. Though Sheen amassed 1 million followers on Twitter in 24 hours, his online show only drew in 150,000 viewers at its peak.
Still, Sheen spent much of the show touting his pop culture prowess.
"It has caught on like an absolute global wildfire," Sheen said of his catchphrases and public interest in his life.
With no regular show to shoot, Sheen seems to be focused on building his online brand -- as one of Ad.ly's celebrity tweeters, he could make $1 million a year with sponsored tweets.
On Monday afternoon, Sheen tweeted a job opening for an intern:
"I'm looking to hire a #winning INTERN with #TigerBlood. Apply here -- http://bit.ly/hykQQF #TigerBloodIntern #internship #ad"