Once a person reaches a state of flow with a task (fully focused and owrking optimially) once they are interupted it typically takes Twenty three minutes for them to get back in to the task and 30 mins to get back to that peak performance “state of flow”, basically every time you interrupt me you could take me an hour to get back to where I was before interuption. The scary side is they found there is a One in Eight chance I wont go back at all that day, thus diverted away fromw whats needed.
If someone is working, unless its really needed, and you cant make progress alone, leave them alone or wait until they are on a break. Naturally common sense applies.
Here is a great link to an blog post by Steve Pavlina on interruption and its impact on a persons productivity and ability to work. This goes with the general rule of slicing people time, every time you slide a person between projects you lose 10% of the net capacity and this explains some of that in more detail. its also interesting that there is a 1 in 8 chance the person you disturb does not get back to the task at had on the same day.
This really enforces the Agile view that tasks should be short and focused to people can get things done, that establishing a rhythm is good so people can plan there time without interruption and that having a person such as a scrum master whos job is to help prevent interruptions is a high value role.