Bean has two guarding modes, depending on how enthusiastic he is about Potato's request to cover him. Either he naps somewhere near the guarding point, with his head on the ramp, roughly in the right place to say he's guarding. Or he stands in Potato's position, wildly whipping his head around, I guess when he's feeling more nervous.
Potato, on the other had, just stands there motionless, like the British Royal Guard.