[mcv]

Having to put it in writing is still an extra hurdle. When someone is sitting next to you, it's easy to nudge him and ask "what do you make of this?". When he's somewhere else, you may have to paste or point to the code, possibly include screen shots, figure out how to give him access to the code running locally on your machine, etc. These are significant hurdles for really quick, short interactions.

Also: pair programming.