The wheels of business stop for no one, and when an employee goes out on leave—or otherwise—it's surprising how quickly the inconveniences mount. The urge to pick up the phone or dash off a quick email to ask a simple question just to keep the wheels moving is powerful, but pause for a moment before you do it and ask:
1) Is this employee the only one who can provide us with the necessary information? If the answer is yes, that might actually be a bit of a red flag. Perhaps information should be shared more freely before a single vacation or stretch of sick leave causes a crisis.
2) If this project simply stalled until their return, would it be the end of the world? We tend to see life through work-colored glasses, believing everything is critical, and forget easily that many issues can indeed sit for a while without devastating impact.
3) Has anyone else beat me to the punch? A rare call to take care of a pressing issue is usually no big deal; a pattern of them, though, can give the appearance that you're not letting the employee maintain the separation from work to which they're entitled. Make sure you're not adding to what may already be a growing legal concern.
4) Is this employee easygoing enough to forgive the intrusion? This may be the most pertinent question of all. You need to have a feel for which workers seem perfectly amenable to being called or emailed when they're away and which do not. If the one you're reaching out to falls into the latter category, seek a workaround. Interruptions to leave tend to linger in the memory and resentment can build quickly.