There's a lot to do in Red Dead Redemption 2, but don't forget to take a break from bounty hunting and card collecting to simply run at top speed into buildings to scare the crap out of the people inside. There's a montage above (also here on YouTube), which I suggest watching with the sound on to hear both the sounds of the doors slamming open and the various shouts of surprise of the people behind them.
This new hobby of mine developed after I randomly ran full-tilt through some stranger's door and into their house. The door flew open in a crash and the poor fellow, who was innocently sweeping his floor, ducked his head and yelled "Oh my God!" I was so happy the game reacted to my abrupt entrance that I made it a point to crash into this guy's house every time I passed through town. And then I started doing it with every door, in every town, every time.
There are a few hurdles to achieving good door-bustin'. Firstly, most doors in RDR2 don't even open and you're going to wind up with a lot of bruised shoulders (well, two of them). Merchants are no fun—no matter how hard you smash through their doors, they're always calm and happy to see you, even if you shatter the glass on your way in. And an enormous mansion I found has dozens of rooms, each with a door to burst through, but sadly no occupants to startle. Disappointing!
For an added challenge, try doing it while in cinematic mode. It's tricky! And take note of any rooms that have more than one door. You can smash through one door, scare someone, then quickly run around outside to the other door, smash through it, and scare them again. Unfortunately, trying to get my horse in on the fun didn't work so well. Sorry, Stumpy.