In other words, you can change from passive voice to active voice by making your character the subject of every sentence, thus placing emphasis on that character.
No. Your character could easily be the subject of a sentence with a verb in the passive voice.
"Choose between an active or passive voice based on what you want the subject of the sentence to be," is good advice, but it's incomplete.
The passive voice is an aspect of the
verb in the sentence; it is not an aspect of the meaning you're trying to express with that verb. Compare:
verb = give.
"Give" expresses a social transaction that involves the movement (literal or figurative) of an object from one person to another.
George gives a present to Bill.
You can now make "Bill" the subject of the sentence. To do so, you put the verb "give" into the passive voice.
Bill was given a present.
You can also make the present the subject of the sentence:
A present was given to Bill.
So you first choose what you're talking about (the subject), and then the relation of the subject to the verb determines the verb's voice. If you're talking about George, you have active voice; if you're talking about Bill, or the present, you have passive voice. This is the grammatical consequence of the subject and the verb you have chosen.
Now, I said it's about the verb and not the meaning you're expressing with the verb. Why? Because sometimes you can use a different verb to express the same set of circumstances. So:
If you assume the verb "give", and you make "Bill" the subject of the sentence you have the passive voice:
Bill was given a present.
But you could use a different verb to express the meaning, say "receive".
Bill received a present.
"Receive" meaning what it does, the verb is now in the active voice. Putting "the present" into the subject slot would still render the verb in the passive voice:
A present was received.
Note that the verb "receive" does not allow George in the subject slot at all. If you choose the verb "receive", the only way to put "George" anywhere near the subject of the sentence is a rather roundabout construction:
It was George from whom Bill received a present.
Again, you can find a new verb to focus on the present, say:
A present exchanged hands.
Now, both George and Bill are deleted from the core meaning of the verb; though you can add them in a prepositional phrase:
A present exchanged hands from George to Bill.
So, very often, if you really want to avoid the passive voice, you can do so by finding the appropriate verb. It's just that - often - there is no reason at all to avoid the passive voice.
"Bill was given a present," (passive voice) is as precise or vague as "Bill received a present." (active voice) Bill is as active or passive in either sentence, too. Between these two versions, the difference are mainly word count, rhythm, register.
Another, more subtle, difference is that the passive voice "was given" draws more attention to the "giver" (who is often absent from the sentence), than "receive".
To illustrate that difference, take a pair of verbs that make for different slots in a social transaction, say "buy" and "sell":
I bought a book. --> I was sold a book.
See the difference? The active voice version of "buy" focusses entirely on you and the book. The passive voice version of "sell" focusses on you and the book, too, but it also hints at the absent seller.
It's this difference that earns the "passive voice" the epithet of "vague", even though "I was sold a book," is no more vauge than "I bought a book." There is a seller in both cases, and it's mentioned in neither. However the
existance of the seller is brought up if you use the verb "sell". It's in the verbs very meaning.