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Believe it or not, the man on the right really is Jason Segel.

The "How I Met Your Mother" star was spotted in Boston this week, where he was looking dramatically different and noticeably thinner.

The actor is currently filming a new movie with Cameron Diaz called "Sex Tape," where the two try to track down an intimate home video they made and then lost.

Given the title and subject nature, you can probably expect to see some nudity from Segel, which help explains his weight loss.

"I'm not in shape for no reason," he told Us Weekly about his new physique. "This time when I take my shirt off, I've made a promise to myself, it doesn't have to be funny."

So, how'd he do it?

"I've just been living healthy, exercising, eating right—no more midnight pizzas," he says.

How do you think he looks? Check out more stars who have gained or lost weight for a role in the gallery above!

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