One of the highest compliments you can give a cosplayer is to be convinced that they're actually the character they're portraying.
Tons of hours of work go into crafting the perfect costume, hair, and makeup. When you see those fictional characters you've loved in movies, TV, comics, and video games walk around in real life, it's a thing of beauty.
But some cosplays don't require that much effort. Some cosplays are intended for the low-key designer: they require just a few choice details, and, of course, a set of huge balls to carry out the character all the way. It's simple, but that's what makes it so difficult. Working on those simple ingredients require a lot of effort.
Just goes to show you that if a crowd of people go gaga over someone who really looks like Robert Downey Jr. when he's playing Tony Stark, people will join in on the fun.
I'm just wondering why no one took a second to question why RDJ was walking outside in character?