Even in season one there were some times when he let a very different personality shine through. Like that thing he said to Dianne, and the conversation he had in the ally with Todd.

Also, note when he punches the mirror. I think that hints to more Bojack style self loathing then anything, he's just much better at hiding it.

http://40.media.tumblr.com/… Is what slothes look like in Bojack Horseman