Funnyman Jack Black has vowed to stay sober while working on his movie sets - because he's too "paranoid" to take drugs or alcohol.

Black, who plays a drug addict on the set of new war comedy Tropic Thunder, admits he admires his former drug-riddled co-star Robert Downey Jr. for once being able to work under the influence.

But he insists his own sobering experience in acting class as a teenager prompted him to kick his bad habits while on the job.

He says, "I had an improv class in high school and I came stoned one time. I was so paranoid that I never tried to f$%k with it again.

"I marvel at some people's ability to party and then act. I'm not built that way."

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(This news article provided by World Entertainment News Network)