An opinion exists that overusing of the passive voice can make writing harder for reading and understanding. Is it true for all kinds of writing? How to follow this advice without overemphasizing it?
Is there ever a good time to use passive voice?
Mark the passive use in your text. Read them aloud. Change the sentences to active. Read them aloud. What sounds better?
With the passive form it is harder to visualize your story to your reader - less "action". "Philip was killed by Martin." sounds less dramatic than "Martin killed Philip". What picture do you want to create in the head of the reader? Answer that question and you can decide for each passive use, if it is appropriate or not.
With active voice you can drive a story. Passive voice often sounds like you are just a bystander, watching something what you're not involved to.
In fiction, I find active voice will usually lead to a more direct connection from the characters and action to the reader. Passive voice can be useful when the writer wants to demonstrate emotional detachment or distance. Re-casting a passive sentence in the active voice can occasionally lead to a convoluted mess; in these cases, I'd just write the sentence passively (and clearly) and move along. Unless you're generating business, technical or legal writing, you'll get better results by using active voice more often than not, particularly paired with a show, don't tell approach.
(Someone will produce a masterpiece of emotionally intimate fiction written almost entirely in the passive voice now, I expect.)
What do you want to emphasize? If it's the person doing the action, by all means use active voice. "Heinrich Schliemann ate a sandwich" emphasizes Schliemann, while "the sandwich was eaten by Heinrich Schliemann" emphasizes the sandwich. Contrast this with "Heinrich Schliemann excavated Troy", which seems more at home in a biography of Herr Schliemann, versus "Troy was excavated by Heinrich Schliemann".
Usually, the person and the action are the more interesting parts of the sentence, and people identify more with the person than the object. This typically means that active voice is more interesting and easier to read. (This holds even when the object is the more important part, which is one reason scientific papers are often unpleasant to read: "The correlation was found to be significant at p < 0.01" is dryer than "We found the correlation to be significant a p < 0.01", even though the important part is the correlation and not the investigators.) "Troy", on the other hand, grabs much harder than "Hermann Schliemann".
Here's how I think about all writing rules that tell me what I should or should not do: What is the effect of doing this? How does it affect the reader? When would I want those effects? When would I not want them? What effects do I want to create right now?
So what are the effects of passive voice? Passive voice deemphasizes, hides, or ignores the actor and the actor's responsibility for the action. As a result, it gives greater emphasis to what remains: the act, and the recipient of the act.