Why Does David Letterman run across the stage before the screen goes up?

Why Does David Letterman run across the stage before the screen goes up?

According to Tony Mendez, it's because Letterman walks down a long hallway to get there, but because they film in real time, he waits for the music to start before he starts walking. He has to time it to where he's at the right spot at exactly the right time. This is why sometimes you see him run across the stage, and other times he's already there.

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