In AmE, it's more common to make a joke or maybe even a sarcastic remark when you've interrupted someone's work. (They know that you don't mean it in a hurtful way, though. They understand that you're being funny and that you understand they're working hard.)
For example, if you call a friend who works in an office, and he tells you "Oh, I can't talk right now, I have to get this presentation finished in half an hour for my boss." You might reply, "Well, I won't keep you from your fun, then!" Or you simply might say, "I'll let you get back to it then, call me back when you have a chance."
If Dave was outside pruning his tree as you left, you'd probably say something like, "Looks good, Dave. When you're done there you can come over and do my trees!"
- For Teachers