In regards to being a player in an RP as a GM, it is perfectly normal and encouraged. It's typically frowned upon to make yourself the main protagonist and use the other players as sidekicks, but having a model player character in the RP that other players can choose to follow and take cues from is a good idea. I've never seen someone call out the GM for giving his character special privileges. I have seen people call out the GM for treating a player more favorably than everyone else.

As a GM player character, more people will want to interact with your character. You are the one moving the story along after all. But I have noticed a reluctance for other characters to form deeper bonds with my characters when I'm the GM, such as friendship and romance. That could just be my experience though, but I suspect it's because GMs are also very secretive. we don't want to reveal too much to the players, and so they don't trust us as much with deeper relationships.

If you want to be surprised, you need only give your players the freedom to do such things. An example where I was a player, one of my characters were attacked by a vampire-thing. I fled from the attack, and my character said a flirty line towards the attacker, because he's a jerk like that. Turns out that while the monster is nigh-immortal, it is weakened by words of love. So instead of someone stepping in to save my character from this creature, someone stepped in to save the creature. Long term nothing really changed, but it was something the GM didn't expect, but was able to adjust for. Other examples of surprising the GM are hidden backstories. Don't force your players to write everything on the character sheet.