There's an old writing adage that says once a story starts to slow down, have someone burst through the door unexpectedly. This technique may have worked back in the days of pulp fiction and classic comics, when action came first, logic second. But today?
Regardless, it doesn't mean that a well-planned surprise can't work wonders for your fiction, and give you a boost of inspiration as well, particularly if you're suffering from writer's block. Whatever the motive behind your surprise, make sure that it 1) ties back into the main plot, 2) reveals something important about your characters, and 3) comes across as believable, rather than contrived.