Tell them politely and forthrightly. Using your mouth. Otherwise, you'll be pegged as an e-mail coward. Don't talk business in the bathroom. It puts people in the awkward spot of having to agree with you because they don't want to prolong the conversation.
Managers tend to resent being put in an awkward spot.
They're funny like that. When your boss calls you at home on a Saturday, speak as if you were sitting at your desk. Do not mention that you're taking something out of the oven or that the delivery guy is at the door. But always pause for a moment before asking a senior person a question to make sure it's not something a peer could answer. A new hire who was junior to me once went straight to the director with a question I could have answered easily.