Gotham's Guide to Playing Canons
It can be tough to know how to handle playing a canon character, especially if you've never done it before. This is the process I go through when preparing to play one, as well as a few of my views on the matter.
Step 1: Pick the Canon
The first step in playing a canon is, of course, picking the one you want to play! I never decide to play a canon because I'm personally attached to them or like them a lot. I pick canons that spark an interest in me, a desire to learn more about that kind of mindset. I also pick canons that will challenge me in some way.
The reason why I do not pick canons which I'm super attached to is much for the same reason that you're advised to not pick your best friends as project partners. Also, I like to see my most favorite canons played by other people so that I have a chance to interact with them as well.
Step 2: Reduce to the Simplest Component
Next, I reduce the character down to their smallest part. When everything else is pulled away from the character, what is the one thing that they are left standing with? What is their main concept? What is at their very core? If they were burned away to nothing and only one thing was left, the one thing which would rebuild them entirely- what would that thing be? This is the thing that will create the shape of the character as they go about their life.
In order to perform this reduction, I consume all the media I possibly can about the character. I watch very closely what they do, how they do it, what they say, and why, and understand that it all has happened for a reason and that there is meaning in every interaction, no matter how small. If there is conflicting information on the canon, I personally choose to accept both sides of it into the character and figure out why this person would react in two completely different ways. Just because it is surprising and doesn't seem to fit doesn't mean that it's impossible or even out of character: everyone has their off days and breaking points.
What this simplest component is usually hits me while I'm taking a walk or having a shower- doin' all my critical thinking! Examples of what these simple components are are: a desire to prove you wrong, thievery, perseverance, determination, and duality. It is possible that this component seems to contradict the rest of the character, but remember that people do strange things to protect their softest parts, and ugly scar tissues formed when wounded.
Step 3: Put Them Back Together
Once I've found the smallest piece of the character, I put them back together and try to figure out what other traits of theirs formed next. I feel that each piece begins to make more sense as it falls into place, and it's much easier to understand how the character went from who they were born as to who they are now.
Of course, everything doesn't have to make sense right away. It's all right to still have some questions about things. As you play the character more, the answers will become apparent.
Step 4: Identify with Something
I play a lot of bad guys, and it is important to me to make them appear human. I never want to play a mustache-twirling villain who is so shallow that they are evil just for the sake of being evil. I don't believe such a person exists. And in order to convince a reader that this character is human, I first have to find that spark in them myself.
If I have a hard time with this, I just remember that this character also has to do all the things I do: eat, sleep, use the bathroom. They've cried sometime in their life, and have laughed so hard they couldn't breathe. They've been sick and hungry and sad and angry.
Of course, finding something you can identify with in a character is a good strategy even if you aren't playing a bad guy. In general I think it helps to portray a more convincing character.
Step 5: Fake It Until You Make It
100% of the time I am 100% way too in to my own head, and am constantly worried I'm not going to be able to pull this character off in spite of the [literal months of] research I put into the character. But have confidence in yourself! If you are playing the character truthfully and are not playing them only for wish fulfillment purposes, then you'll get the hang of it! You don't have to be perfect the first time, it's okay! It'll click into place after a little bit of posting with them.