Cross Processing in still photography - shooting with
E6 (slide) film and then processing as
C41 (negative) film - can produce alot of wonderfully interesting effects.
Like
Yossarian describes, the negative will have intense contrast. So much so, in fact, that it is not good for the fine detail of a person's expression unless the blown out highly contrasty and grainy look is a part of your desired result. It is, however, unbelievably beautiful to cross process images showing hard and varied textures, or anything else you'd like to see starkly portrayed. Even nature images look amazing and different, although somewhat surreal, using this technique.
Your negs will not always have an overly yellowish hue; different slide films have different effects when cross processed and experimentation is recommended. The colors may range from normal to somewhat strange to completely off the wall, and are almost always overly rich and vibrant. This can help with subtle things like pulling a
brilliant blue sky out on a bland gray day, or achieving
magic hour shadows in a drably lit situation. Much more fun, though, are the amazing and powerful
artsy effects you can get with the overly-brilliant colors produced.
Kodachome and
Ectachrome, when cross processed, do lean towards rich and warmer yellows while
Fuji slide films (like the cheap and readily available Velvia for instance) can bring piercing greens and blues. The contrast across the board is always extra intense.
And yes, cross processing is also often used in moving picture environments. You can see its effects all over the place, from some of
the overly-attempting-to-be-gritty-and-urban commercials on MTV these days, to the occasional
hollywood usage (the yellowed and strangely stark Mexico scenes in
Traffic can be attributed to this technique, as can those purposeful blown-out colorful scenes appearing in almost all
Spike Lee flicks.)
Cross Processing is also a term used to describe combining multiple techniques for direct positive-to-paper
experimental photographic imaging. For instance, slathering a canvas with brownprint (a photographic
emulsion that can be applied to almost any dry, natural surface) and shining light through a
lith film positive directly onto the surface to create a negative image is called cross processing.