I've blogged several times about Startup Therapy.  This is the idea of spending time, as the startup is developing, for everyone to understand what each others expectations are.  See previous posts such as the The Emotional Life of Entrepreneurs, Startup Therapy - More Thoughts and No Lost Founders for earlier thoughts.

Just last week I was talking to a budding entrepreneur who has a full time, and highly visible, position already, but was working with a colleague on a startup.  They had agreed, but not in writing or in legal docs, to a 1/3, 1/3,1/3 split, including an engineering colleague.  One day, "our entrepreneur" went to his colleague who said, well you're out of the company, the engineer and I have been working on a 50/50 split for several months. 

Needless to say, this was a shock to our entrepreneur, especially as he had been instrumental in developing many contacts for the budding company.  Their relationship is in ruins, and obviously that's one highly placed person who is not disposed to help this emerging company!  Even worse, the two had been close friends and also worked together in the day jobs. 

If you feel that one partner in a venture isn't pulling their weight, or there are other issues be up front and say so.  Have the conversations, work out various options, but don't just "spring it" on someone - that destroys trust and relationships, not to mention being just plain wrong.