Cropping all layers of an animated gif

[digital_vibe]

Having imported an animated Gif into PS, I want to use the Elliptical Marquee Tool to alter the shape of the gif from a rectangle to a circle (with a transparant bg). I'm sure there must a simple way of doing this but all attempts thus far have proven less than successful. I can edit a frame at a time but that would take an absurd amount of time and the results would likely be inconsistent. 

Any tips would be appreciated. 

[c.pfaffenbichler]

You could add a Shape Layer, move it to the bottom of the Layers, select all the other Layers and 

Layer > Create Clipping Mask