"Speak of the Devil" simply means when a person mentioned in the current conversation happens to walk into the room.
Example of use: "We were huddled around the tv, talking about Fred, when he walked in. Well, speak of the devil!"
Interesting fact about Speak of the Devil
During the 19th century, this pharse used to be a warning against eavesdroppers "No good of himself does a listener hear, / Speak of the devil he's sure to appear."
