Following themed sideblogs is my #1 "making Tumblr fun" hack! The best way to find themed sideblogs is to search the tags for the character or pairing you're interested in, reblog a few of your favorite gifsets from that, and then check out the blog suggestions that come up in the right-hand sidebar. When you reblog things from those sideblogs in your dash, it'll give you even more suggestions. I have found most of the sideblogs I'm following that way.
Checking out the character/pairing tags periodically, and then clicking on the notes for any interesting-looking text post, has also helped me find the people who are particularly chatty in my fandoms, who tend to be the most responsive and fun to talk to. I figured this out in Agent Carter fandom - lurk on the tags, then explore the blogs of the most interesting-seeming people who tend to pop up most often in discussions - and found that it was equally effective in Flash fandom and helped me figure out who to interact with in a fraction of the time it took me to get a feeling for the Agent Carter Tumblr fanosphere, when I was still learning these things.
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Checking out the character/pairing tags periodically, and then clicking on the notes for any interesting-looking text post, has also helped me find the people who are particularly chatty in my fandoms, who tend to be the most responsive and fun to talk to. I figured this out in Agent Carter fandom - lurk on the tags, then explore the blogs of the most interesting-seeming people who tend to pop up most often in discussions - and found that it was equally effective in Flash fandom and helped me figure out who to interact with in a fraction of the time it took me to get a feeling for the Agent Carter Tumblr fanosphere, when I was still learning these things.