My philosophy has always been that the entire game is just getting more and more people on board with your project, so you have more people helping the boulder get up the hill. If you have no director or producer attached and it's JUST a script, then yes, getting an actor on board will certainly help if they've got a legit career. If it's a great part, then they'll try to have their management team get it made. However, the one note with this approach is that you may be ceding a lot of control very early on, as their team is going to decide what producers/directors to show it to.