Backseat Gamer?

So while watching my brother play through Devil May Cry 4 I was faced with a recurring dilemma. How much assistance/advice do I provide him with as he occasionally struggles with parts of the game? This problem really comes up for me anytime I watch someone play a game that I've played before them. I think it really depends on the person. Over the years, me and my brother have watched each other play through a large number of games. Our understanding has generally been that we give liberal amounts of hints/tips to make each other's gaming experience smoother. I've steered him through many a game over the years and he's done the same for me. I think this understanding might derive from us often effectively co-oping our way through single player games in our younger years. We're used to and okay with helping each other.
For other people, hints are less appreciated in most cases. The most extreme case that I've experienced so far is from my roommate, who apparently hates any commentary on his playing of a game at all. This has made it tough for me at points to watch him play stuff. Nonetheless, I have to respect his wishes to solve a game totally on his own.
A subset of the giving advice aspect of games, is giving advice that is totally wrong. It is surprisingly entertaining to get an unexpected victim to catapult themselves off a level in order to get a secret item or holding down extra buttons to get a "buff". To get this to work you really need to be known as having very good and reliable videogame knowledge. Thankfully, I easily meet this qualification.