From my own experience I think that a lot of times this problem is a lot like nobody wanting to ask a question or answer a question in class. A lot of players new to tabletop gaming are nervous to roleplay or aren't familiar with the right time to jump in. So while they are sitting there playing a Half-Orc Barbarian 2 in their head he might have a disgruntled ex wife, three kids who left the family barbarian business to be wizards and a secret love of chelish opera but you would never know. So the best thing to do at time is just keep playing a character with character and show them how it is done. Sometimes you just have to draw people out a bit to help them be comfortable with the concept of a character actually having character and I definitely have to echo that table tents can be super helpful.